Well, this is my first guide...well...ever. I play Nasus often and I personally think that he has the most potential for awesomeness. I play him as a tank because his pushing power is phenomenal with Spirit Fire and Siphoning strike can be used to quickly dispatch turrets. But this does not mean that you will not get kills. In fact, with this build, you can kill Baron all by your lonesome. I hope this guide will be helpful for anyone who is interested in playing Nasus and feedback is truly appreciated.

Pros

Great at pushing lanes
Lots of health for team fights
Can burn through half of a champions HP with a single hit
Soul Eater makes him almost invincible (In theory)
Can burn through a minion horde with one Spirit Fire
Has the potential 1v1 kill all champions
Has the best voice in the entire game (No, Taric's voice is not as good)

Cons

Runs out of mana quickly early game
Tends to be kited
If you are not careful, you will be ganked often
Slow. Super slow...

Siphoning Strike: Using this ability right is the most important aspect of playing Nasus. It is imperative that you ALWAYS last hit with this ability. It does not matter if it is monsters, it does not matter if it is champions, it does not matter if it is a turret or inhibitor, last hitting makes this ability stronger. There is no max damage that it can reach, it will continue to become stronger and stronger. By end game, if you have played this move right, you should be dealing over 500 damage per strike. This move will make or break your game, so focus on it. I max this last because it will become more powerful by itself.
Wither: This is a really effective slow, probably the best in the game. It helps if you are chasing, running, or fighting an enemy because it slows their movement and attack speed, making Yi or even Warwick not as damaging and a whole lot more kill able. I max this after Spirit Fire because it helps more later game than early game.
Spirit Fire: This is your spam move. You launch this to eliminate a wave of minions, you use it to make a barrier, you use it to finish off champs, etc. It is so useful that I max it out right away. Not only does it cause initial damage, but it does lasting damage and reduces armor. Coupled with wither, it makes champions prime candidates for Siphoning strike. Plus, at level 5, it will kill caster minions with one use, so it is great for farming. Also, if an enemy is about to die, but our of your reach, NO PROBLEM. Just use Spirit Fire and they are dead. Also, if you are about to die, use Spirit Fire to give a parting gift. They will be hurt, and your teammates can come and make the kill.
Fury of the Sands: BLARGH!!! This is your scary ulti. You grow huge, get strong, people around you get hurt, and you get more life. Simply put, it is awesome. However! People use it too early and lose its effectiveness. Only pop it when you are fully engaged with an opponent. After that, it is timing. You do not want to do it too soon, or they will run away with nearly full HP and your ulti is wasted, not too late or they will just kill you, but about when your HP hits half. Then you get most of it back and they are weak enough for you to destroy.

Now, I have played around with Nasus and his build, but this is the one I find the most effective. You can change some items, but here are just the ones I use and for this build. It is reccomended that you buy them in this order, but you can choose for yourself. I will not say "BUY FIRST TIME BACK" or "YOU MUST GET THIS AT THIS POINT IN THE GAME" because **** happens and you may not have enough when you think you should. Just work on buying as the game progresses.

Doran's Shield: This is always a great starter item for a tank. Although it burns all of your gold, it will give an advantage HP and HP regain wise in the early game. You will sell this around the time you get Warmog's Armor
or
This is really a situational choice. I usually go Mercury's Treads simply because of tenacity. With it, debuffs will never slow you down (Metaphorically, of course. They will work, tenacity just makes them work less). However, Ninja Tabi does have extra dodge and armor, which will be more helpful if they have mostly melee champs.

Sheen: This is just a great item. It is cheep and effective. It will also cure your "Oh noes, oom again!" problem. And the extra power after every ability is super useful.

Aegis of the Legion: Here we are. The item that every tank has. IT IS JUST GOOD. Its aura will aid in team fights, the health will make you look scary as you charge in, and the armor and magic resist will help against those annoying enemy champs. Plus, it is pretty cost effective.

Warmog's Armor: This item seems to be made for Nasus. It starts out already with great tank bonuses, but then it stacks per kill. Just by farming a single lane, you add on to its effectiveness. Your Spirit Fire is already amazing for clearing lanes, but now you get bonuses for using it? Sweet.

Sunfire Cape: I really just throw this in there for fun. Extra health and armor is really nice, and the 35 damage it deals to enemies around you is nice for 1v1 or team fights. But its real awesomeness is when you pop your Ulti. Already they are being shredded by its passive, but now they are being hit with the damage from this as well. Now anyone near you essentially dies.

Guardian Angel: This is just an essential tank item. Not dying is important for tanks, and this pretty much covers that minor issue.

Trinity Force: Ah yes, Trinity force. Such a great item. It adds power to basically every aspect of your attacking game, plus it can slow enemies and has the power of Sheen. It just works.

Greater Mark of Desolation: I like this rune because, well, people tend to stack up on armor when faced with a melee tank. It is just fact. With this rune, you can bypass that so you can rip them apart with your attacks.
Greater Glyph of Shielding: When playing as Nasus, ranged mages will try to take you out while you try and catch up to them. To stop this harassment, you need magic resist.
Greater Seal of Clarity: With this build, unfortunately, mana can be a problem. You will be using your abilities often, so you need mana regain. The reason I did not do flat mana regain was because Nasus will need more mana later in the game when team fights are more important and being oom will spell the end for you and your team.
Greater Quintessence of Fortitude: Tanks need more HP. It is just that simple. Plus, it will stop you from dying early game, so that helps too.

As usual, feel free to choose other spells. These are the spells I go with at all times because, well, you will see. You may use whatever spells you see fit to use with this build.
Ghost: Nasus is slow. He cannot flee or chase, and sometimes needs a boost. That is where ghost comes in. While flash gets you away, ghost will keep you moving away. It also helps close the gap when chasing so you can cast wither to finally catch up.
Teleport: As a tank, you sometimes need to get from point A to point B very fast. But you are slow, that is why teleport is essential. It will also help you early game when you need to get back into the fray right after you recall to buy or heal.

Well, there you have it. My first guide and favorite build for Nasus. Comments are greatly appreciated and I hope you have fun slaughtering your enemies as Nasus. Please comment, favorite, and up-vote!