28/08/11 - Fixed a few minor spelling issues.
25/12/11 - Made a lot of changes, major build revamp & mastery revamp.

This guide mainly focuses on Nidalee after the patch(Some may call it a nerf) And focuses on her taking solo top whilst one of her team mates jungle. This will instantly put you ahead in terms of level, farm and gold if you're any good at last hitting (Which you should be anyways if you're taking solo top)
To Carry as Nidalee you need a mediocre team atleast, because unlike other carries she doesn't run in with lolyamag and pentakill their team within 4seconds. Nidalee carries by keeping her team alive with well timed heals and AS steroids, by spearing the squishy carry from far away before initiation to ensure they die quickly and by doing somewhat decent damage in cat form.
Although I've seen this said in guides before, I feel the need to say it in mine too, Nidalee is somewhat like a Feral Druid in WoW(Which means if you're dropping low on HP in a 1v1, don't be scared to drop form, heal yourself and give yourself the AS steroid, throw a spear whilst your cat is on CD then hop back in cat, it can sometimes win the fight), which was the exact same thoughts I had when I first started playing Nidalee, that and she struck me as an Amazon from Diablo 2, but I digress.

Viable summoner spells
Teleport will help you get back to your lane quicker, if you're taking solo top you don't want to leave them alone for too long while you have to recall. It can also help with ganks, not missing teamfights and teleporting to other lanes to support allies in fights.

Ghost is one of the obvious choices for a get away, or even a get-to when you need to get somewhere that little bit faster.

Pretty much the same as Ghost 'cept you can juke bushes and Flash through walls. This nifty little spell will stop your enemies from getting away when you need to Flash+Spear to finish them off. You an also use it to get away on low HP after turret diving.

The only time I would ever recommend you take Exhaust is if you don't and I mean really don't want to take Teleport/Ghost and you are new to Nidalee. The reasons for this being; Nidalee is a hard champ to escape and once you learn her you will hardly ever lose a kill(This is coming from personal experience) You have your speed in Panther, your pounce and your long-range and hard-hitting skill shot, which will always make sure you get those stragglers once you master how to use it.


Semi-Viable spells
Clairvoyance can help to make sure you don't get ganked, know where their enemy jungler is, check blue/red/dragon/baron, check those pesky brushes when you just know they're going for a gank or for pure supporting issues. The only time I would ever want a Nidalee to use this is if no other support will and you feel you really need it to counter-gank.

Can help for get-aways when you get ganked, for when you get focus CC'd in those late game team fights or simply when your enemy team is CC heavy and you don't want to risk taking heavy CC and losing a team fight beacause of it.

If you feel you need that little extra damage increase for 1v1s or even taking down their carry that little bit quicker in team fights, this little gem could help with just that.

Non-viable summoners
Using this build you won't go OOM, so this spell is pretty much useless.

Although it could help during aces to stop the enemy team from pushing too hard, or keeping your turret up whilst you're out of lane I would never expect a Nidalee to take this spell.

You have some of the strongest heals in the game already on a pretty low CD, why oh why would you use this on Nidalee?

I have never seen this spell used correctly and it would usually only benefit 1/2 people on your team.

For me, this spell is a keep away no matter what champion you are.

You don't jungle.

9/0/21, this is pretty much the 'bog standard' Nidalee mastery build, 'cept it has a little more mana regen and still keeps all the CDR. You can mix it up to your playstyle ofcourse, but I recommend keeping the CDR and MP5 no matter what.

x9, Greater Mark of Insight. The Magic Penetration will help throughout the game.

x9, Greater Glyoh of Force. An extra 27 AP at level 18, it may not seem like much but it can make the difference.

x9, Greater Seal of Vitality. Health is defense against both casters and melee, unlike armor and MR. I also don't think the MP5 seals are too needed with this build.

x3, Greater Quintessence of Potency. Taking the AP/level Quints only gives you ~7 extra AP at level 18, I personally would rather have the 15 extra at level 1 and lose the 7 extra at level 18.

Pros
Amazing sustainability
Extremely versatile (Can take solo mid, solo top, can even duo lane as support/DPS)
Amazing farm post-level 6
Traps act as anti-ganks
Jungle/map control to some degree with your traps
Speedy in Panther form
Mediocre pusher with decent turret damage
Great damage to squishies
Decent heals

Cons
Somewhat difficult to play with a decently high skill cap
Somewhat squishy if caught out of position
Main target in most team fights
Almost useless against tanks

Core Build

Doran's ring makes for a great starter item on Nidalee just like it does on a lot of AP powered champions. The early game HP makes for nice survivability, the little bit of MP5 will help you lane until you get your Tear and the Ability Power will give your spears that little extra damage.

Most people don't go for Tear of the Goddess, but this item really does help with Nidalee's mana issues that came with the new patch, it also builds into Archangel's which is another great item for Nidalee. The mana boost and MP5 boost are nice and will keep you laning forever, but the reason I get this nifty little item so early is the passive; building up this passive early game will help you throught the early/mid game phase a lot, but will still be useful late game too.

You may be wondering, why boots of Lucidity over Merc Treads? Well, I don't usually find myself in gankable positions, or even getting harshly focused in teamfights due to taunts and stuns keeping the enemy off me. The CDR these little gems give you also help an amazing amount, you can heal a lot more often and your enemy will be trembling in fear when they get hit with spear after spear.

Now we're talking, let's get us some stopping power. Rabadon's is an obvious choice for Nidalee, and one that everyone goes for. The 140 base Ability Power it gives you will make a great deal of difference to your heals and your spears, the enemy squishies will be running away as your spears hit for around 30-40% of their HP mid game. The enemy carries will be left confused as you heal yourself and run away from their gank, making it out on a decent amount of HP.

For me, Lich Bane is an obvious choice on any physical damage dealing Ap champ such as Nidalee or Kassadin. This will make your auto attacks hit like a truck, be it in panther form or in human form, if you learn to control the passive on this item you will tear your enemies apart.

Most people will find this item somewhat questionable on Nidalee, but I found that she had some mana issues since the patch, hence us getting ourselves an early Tear of the Goddess, this is just to complete that Tear. The extra AP helps a little, but then the passive helps a lot. Using this build you will find yourself with enough mana to go from ganks to teamfights with no issues, which means turning 3% of that mana into AP will make a notable difference, and believe me it does.

Counter Build
Now we start counter building, we have our core AP, and a decent amount at that(322) So let's build according to our enemies, our playstyle and their playstyle.

A lot of people would put this in their core build, but I find that Nidalee's spear already hits hard enough without this, and besides you shouldn't be focusing tanks and the enemy squishies will still be taking a heap of damage from your spear. We should build void staff when; the enemy are stacking MR, you ain't being focused in teamfights, you don't find yourself needing extra survivability or you want that extra stopping power.

Zhonya's Hourglass is an amazing item, not only supply a great deal of Ability Power but it also brings armour into the equation and a sweet on-use. So, when should we be building Zhonya's; When you find yourself being targeted by AD carries, when you need the extra armor due to AD AoEs in teamfights, if you find yourself dying instantly after the fed Shaco opens on you or if you need some extra survivability but still don't want to sacrifice the AP.

There's already a fair bit of discussion between these two, so I won't help you decide when you need which as I know people will just contradict what I say due to it mainly coming down to personal perference. We build BV/FoN when we need that extra bit of magic resist as the enemy AP carries are hitting us hard, or we want to survive Karthus' ultimate and AoE damage. We pretty much build one of these two, or both if you must but I wouldn't highly recommend it, when the enemy mages are tearing us apart.

Laning Phase

This will be focusing on laning solo top with a jungler on your team, but it will also work for laning phase in duo-lane and solo-mid too. Whilst laning you want to keep yourself up with Heals, but don't go over the top or else you could easily OOM yourself early game. Keep poking your enemies with your auto attack and if they get out of position punish them with a spear, they don't do amazing damage early game but if you hit a few you'll force them in to a recall. You don't want to go for a head-to-head 1v1 until you have your panther form and feel you can certainly get the kill and get away. If you're struggling to last hit before you get cat form them heal yourself for the AS steroid, this will near ensure you get the last hits if you use it right.
Once you have cat form you can start farming effectively and out-farm your opponents in solo-top a lot, just try to not get yourself too harassed, an auto attack of two won't be too much trouble as you can just shrug it off with a heal and lane for an eternity. When creeping in catform you mainly pounce the caster minions and follow it up with Swipe, this will pretty much one shot them and if it doesn't you can auto attack each of them once or twice first to make sure it does and to make sure your enemies can't harass you.

Mid Game

By now they will be no doubt trying to gank you, so make sure you read my section about traps and you should be perfectly safe. At this point you will be in semi-laning phase as I like to call it, you will be leaving your lane for ganks and the occasional team fight, but if you don't have their turret down you will still be attempting to take it down in cat and keep yours alive. Your spears will start hitting for a heft amount here as you will probably already have your CDR boots, Tear, Doran's and one piece of your Rabadon's, meaning you can push enemy squishies out of your lane fairly easily if they move out of position one bit.

Late Game

Late game Nidalee makes for an amazing pusher in cat form, you will have most of your build and will be dealing a whole load of damage with your spears if the enemy even try to defend their turrets from you. Once their turrets are clear of champs and you have a creep wave you can run up in cat and burst through that turret whilst running away safely before the enemy champions even get chance to gank you.

Team Fights

Team fights as Nidalee can be extremely hard. If it's your team initiating on them then first you may want to get out of position a bit and get in some brush beside the enemy team (Over a wall if you can so they can't get to you) And from here you throw a spear or two at the Squishies/carries to make sure they're low health for the team fight. Now you can do one of two things, you ever get your team to run in and take the enemy team from the side(CAn lead to a lot of deaths if the enemy team have any idea as to what game they're playing) Or you can double back to your team and initiate with them whilst staying at the back.
Now, in the team fights it's your job to throw heals to whoever needs them, now obviously who you heal is effected by in-game variables and judgement of which teammate will contribute more towards killing the remaining enemies and winning you the team fight, but the basic hierarchy would be: yourself>AD carries*>AP carries>tanks>others.
*We value AD carries over AP carries because they will generally benefit more form the AS steroid, which is something you must take into consideration when healing in teamfights as it can get your team an extra kill or two and make a great deal of difference.
On top of healing you are also expected to spear the enemies that are making a run for it if your carries can't get to them first and DPS in cat form as you can cause some serious damage at this point with your Lich Bane passive.

Traps

Nidalee's traps are amazing if used correctly and will save you from ganks on several occasions, you should generally trap the edges/coreners of brushes, as this is where the enemy will usually sit and wait for the gank on you. When travelling to other lanes to support team mates or go for ganks make sure you trap the brushes in the river and the brushes on your team mate's lanes, this will help for map awareness and will also stop your team mates from getting ganked within the next 4minutes. Nidalee's traps are also great for warind the brushes near your own blue/red as well as the enemy blue/red if you ever steal them, that way you know where the enemy jungler is and can easily gank him, especially if he repeatedly triggers traps, you will be able to somewhat pick up on his jungling route.

This for example would be used if you was soloing top, the green dots are where you would put your traps to ensure you don't get ganked. The one further down the river isn't so essential and the one in the brush close to your turret is to ensure you don't give dived from the side.

Start placing traps as early as possible.
Keep your vital traps up and refresh them before they vanish.
Don't forget your cat form pounce can jump over some walls, such as jumping out the river.

Thanks for reading my guide, I hope you atleast learned something and won't flame me too hard :')
Constructive criticism is welcomed and I will answer questions.
To come: Opponent difficulty chart and how to lane against different champs.
Talk about blue control, how usefull it is and when to get it.