Nidalee is an extremely versatile champion that can be built in many different ways. I usually build her as a flat AP support. However, I've been playing her a lot lately, and I've been experimenting. As some of you may know, I like hybrid builds, and my only other build is a hybrid jax build, currently the highest rated Jax build.Here's a link So, I decided to build a Nidalee build that focused on AD, but got enough AP to make sure that her AP scaling on most of her skills still did something. I played my first real game with this build earlier today, and went 11-0-6. It was awesome. I laned with a Pantheon and then a Singed vs a Karma and a Master Yi. I gotta say I thoroughly enjoyed myself. So, here we go.

I like getting away. I like not dying. So I have chosen escape spells.
Ghost: If you see a group coming at you, duck into the bushes and ghost and bam you're gone.
Flash: Go through one wall to pounce through another for an instant getaway. Awesome.

Other Spells:
Clarity: Useful, but if you know how to manage your mana, relatively useless.
Clairvoyance: If you think you don't need two escape spells, grab this. It makes for easy scouting and a perfect way to guarantee a max range spear on an out of sight enemy.
Ignite: meh
Exhaust: Useful for shutting down an auto-attacker in a 1v1 or teamfight, but I don't take it often.
Cleanse: A good choice for removing CC, makes getting out of teamfights easier if you get stunned or disabled. I'd just build a quicksilver sash rather than take this though.

Pretty self explanatory.
Quintessences: Flat Health for early game survivability, because Nidalee is rather frail.
Marks: Armor Penetration or Magic Penetration. I find myself switching based on my team makeup.
Seals: Mana Regen per level, for mana regeneration purposes. What else would you put here?
Glyphs: CDR glyphs for more spamming of skills, both in cougar and human form.

I'm not a rune master, so you can really use whatever works best for you. This is just what I use.

Standard 9-0-21 caster setup. AP, CDR, and a beautiful 15% magic pen from the Offensive Tree, and then improved Ghost and Flash, along with mana regen, experience gain, movement speed, and more CDR from Utility.

Doran's Ring: Great starting health, mana regen, and AP. The 15 AP from this and then the 93 from guinsoo's at 8 stacks puts you at 108 AP for a total of like 2600 gold. Along with all the other benefits.

Boots of Mobility/Swiftness: I like being fast. I like crossing the map in seconds. So I like these boots. I usually change my boots from game to game. I get one of these two or sometimes the boots of ionia for some easy CDR/

Manemune: Great source of AD and basically guarantees that you don't run out of mana after you get some stacks up. It's also cheap, which is really nice.

Brutalizer: Sweeet item. A nice 25 AD mixed with armor pen and CDR. Cheap too! This and boots of Ionia put you at 25% CDR, and your masteries get you another 9%, along with glyphs puts your CDR at 39%, almost maxed. You should replace the Doran's Ring before this if you find your item slots full. Don't upgrade it to a Youmou's it's not worth it.

Guinsoo's Rageblade: Love this item. Provides 35 AD and 93 AP at full stacks, and being nidalee you should always be at 8 stacks. Just switching in and out of cougar will get you there before a fight.

Lichbane: This should give you a huge damage boost, along with some great movespeed. The passive effect means a good amount of your auto attacks are hitting with all of your AP behind them, and that hurts. The extra mana it gives should help out your Manemune too!

Rylai's Crystal Scepter: Good late game item, it provides a lot of health, nice AP, and a slow that works with your spears, traps, pounces, swipes, etc. Good stuff, it makes a good chaser even better. If you want more of an AD focus, a Frozen Mallet could be pretty nice too.
Rabbadon's Deathcap: Sell your brutalizer for this. If you manage to get this far in a game, this item is definitely your best choice. Because Nidalee doesn't really shine as a late-game champion, getting more AD won't really help all that much. I'd much rather have my AP doubled, which allows me to switch my role over to a more support champion if the other team starts to focus me when I go into fights. Instead of dying for the damage I put out against full geared up people, I'd rather get some huge AP and focus more on healing my teamates and spearing squishy's until they're about half health which is when you can go cougar and do a W-E-Q rotation to kill them.

Other items:

Banshee's Veil: If the enemy team is caster heavy, get this over a frozen heart, or even with a frozen heart. Good health mana and magic resist, and the passive is nothing to shake a stick at.

Bloodthirster: Great damage and lifesteal

Hextech Gunblade: Good choice if you want more AD and AP, and the lifesteal and spell vamp are pretty awesome.

Early Game: Pick up your Doran's ring and head out to lane. I would recommend taking solo or mid, but if you can't do that take a lane with someone you think you can do well with. Tanks that can disable or slow people are good, and any lane partner with CC is great. Pantheon + Nidalee worked really well for me, especially against a squishy lane. Focus on getting last hits and occasionally harassing with your spear or auto attacks. Remember to keep your health topped off with you're heal. Make sure you're not spamming the spear, unless you're hitting distance spears every time. Otherwise it's a waste of mana you might need later.

Mid Game: Go back once you have 2k+ gold. Pick up your full boots along with a tear of the goddess and a longsword if you can. If you have over 3k (unlikely) get both the boots and a full manemune. Start ganking and really pushing lanes, that's something Nidalee is really good at. You should initiate with a spear, and then heal yourself or an ally then jump into cougar form to do a W-E-Q combo, and if you think you can finish them off with another rotation, do it unless it's too risky. If it is, pop back into human form and try to Q them or auto-attack them down. Most likely they're dead by then anyway. A well aimed spear can end the life of that guy running away in a straight line with 200hp. Even if they move, most players lean a specific direction, so you should be able to lead with it pretty well. you'll get better at it with practice.

End Game: You really start to lose importance here. Other players can wreck you endgame, and cancel out your damage. Focus mostly on healing and poking people, but always be ready to pounce at someone if they're squishy and at half health. A w-e-q combo in cougar form will almost definitely kill them. Backdooring is always fun too. If you know where most of their team is, don't be afraid to backdoor. If they're coming, your traps in the jungle should alert you to them, and even if they don't, a flash+ghost+pounce should get you away, hopefully with a destroyed tower behind you.

Don't be stupid. You're not a full carry. You can't run into a team fight and expect their team to die, in fact if you do that you will probably die. Just play smart, and know how to pick your fights.

TRAPS: Put them everywhere. Treat them like teemo's mushrooms. They allow for great map awareness, and laying them down during a fight is great because the armor/magic resistance it takes away from whoever steps on it really provides a damage boost for your team.

This build is something you should do based on your team makeup. If you feel like your team could use some AD along with some nice heals, go with this build. But, Nidalee is a situational champion, so you should really only use her if you feel like she fits in nicely with your team. Don't use her along with another support champion, because chances are you won't have enough late game damage on your team.

Practice Makes Perfect! Nidalee is a hard champion to play. You really need to play a lot of games with her to get the hang of it. Don't play her for the first time and expect to do well. You need to play a bunch of practice games with her to get a feel for it, and then transition into normal and then ranked gameplay. If you can play with some buddies, that's even better because they should be able to point out things you could do better.

Good Luck!