Frozen Heart - Should be your first buy after Trinity force. Very good armor item.

Force of Nature - Get this if you face an AP that causes troubles for you. Your best source of MR.

Quicksilver Slash - Cheap and good MR source. Counters a lot of debuff ults and supressions.

Aegis of the Legion - If your support or jungler doesn't want to get it, you should. Gives good mixed protection.

Spirit Visage - To counter bruisers that do magic damage. You should get this in lane.

Guardian Angel - *DING* extra life! Gives good mixed protection.

Warmog's Armor - Against true damage *****s. Quite costy, but you can stack it up easily.

Atma's Impaler - Good synergy with Warmogs and your ult. Makes you hit like a truck.

Banshee's Veil - Against combo-casters. Not the best, but viable.

Thornmail - You fed Trynd, You fed Yi, the enemy stacks AD (note the rhymes).

Randuin's Omen - Can help out your ult to keep enemies around, also good for countering ranged AD.

Black Cleaver - Good if you want more utility, I don't like it, but it works.

BloodThrister - With this, you can 1v1 anyone. Be sure to stack it up first.

Youmuu's Ghostblade - You pretty much dominated your lane (rare but happens), and you want to abuse it even more.

Last Whisper - Best offensive item other than Trinity force. Must buy if the enemy team is stacking armor.

Mercury Treads
You'll use them most of the time. These are the best (and most expensive) boots in the game, I just love them. There's no reason not to pick these, you can't go wrong with the magic resist as you'll face some magic damage in every game.

Ninja Tabi
Tabi is a worthy purchase too. Nasus stops benefitting from Mercs once endgame pops, because at that time, you'll be tanky enough to survive through chainstuns, and if you still get cc'd, two carries will still be free to deal damage. Magic damage has never been a big issue for Nasus, as AP carries won't waste their precious cooldowns on tanks (ideally). I'd say go for first, but switch it to later once you feel crowd control doesn't endanger you that much (about after 40-50 minutes in).

Ionian Boots
CDR boots. You can get CDR from better sources on a tank Nasus, I don't recommend using these, you'll be weak to CC early on. Only use them if you don't want to implement Tanky CDR into your build.

I see a ton of questions about who counters Nasus, etc. on the forums. I find this fairly ironic considering 75% or more of the common top lane roster is favored in a lane against Nasus, and at least a dozen can completely shut him down. Also I always see incredibly stupid **** in these threads like "Irelia hard counters Nasus" or "Cho wrecks Nasus". After seeing these threads for months I've grown tired of some of the misinformation spouted and decided to add a list of all the common top lane champions. I will briefly break down each lane, items to buy, tips and tricks, and and over all rating of the lane difficulty. Final conclusion on the lane will be given in the form of: Favorable, Challenging, and Get Wrecked. Why not just use Easy, Medium, Hard? Well, that would be pretty misleading. Even the favorable lanes I would not consider "easy", as you don't really beat anyone at all in the lane. Likewise, the challenging lanes I wouldn't consider medium, I would consider them hard, and the hard would be practically impossible. So I guess you could say the scale is Medium, Hard, Impossible, but whatever, I'm trying to be original.

Please note that this list is not a guarantee that you will win or lose said match-up, as your competence and the competence of the enemy factor very largely into this. For example, I have beaten a great deal of the "Get Wrecked" champions just because they were unaware of how badly they could have shut me down and instead sat there in a farm war with me. Similarly, I've given up first blood to Jarvan, been forced out of lane and outfarmed by Talon, etc, either because of great play on their parts or poor play on mine. These lists merely indicate how the match-up SHOULD go if the champions are played properly and to their full potential. Also, I will periodically update these descriptions and ratings based on personal experiences.

Favorable
Favorable means this is a rare gem of a lane that Nasus should be able to free farm in. Don't take this to mean you can "beat" these champs and try to fight them or force them out of lane. That's not the point. Favorable means they don't have the skill-set to zone you, force you out of lane, and keep you from farming Q. They can still be tricky, put out some strong harass, and remain a threat, but typically these lanes are farm wars, and if you go equal in farm in any of these lanes, you have actually won.
Champions: Cho'Gath, Galio, Irelia, Jarvan, Malphite, Nasus, Singed, Talon

Challenging
Challenging means the lane will be, to varying degrees, a very difficult uphill battle, but is typically manageable. These lanes should generally be avoided, but if you have the runes for them, have experience with the match-up, or are just a glutton for punishment, have at it. With some early pressure from your jungler, these lanes should not be a complete lost cause. Results guaranteed to vary.
Champions: Gangplank, Garen, Jax, Lee Sin, Mordekaiser, Nautilus, Nidalee, Pantheon, Renekton, Riven, Rumble, Shen, Shyvana, Tryndamere, Volibear, Warwick, Wukong, Yorick

Get Wrecked
Get Wrecked means you either rolled the dice with Nasus in blind pick and got really unlucky, or you got hard countered by their last pick and are going to get your **** pushed in real good. Or you are really just that dumb and picked into it. Avoid these lanes at all costs. If you find yourself in one of these lanes, beg for ganks or a swap and try not to feed. Otherwise, consider letting your tower fall and hope they will go away and let you farm at your second turret.
Champions: Akali, Kennen, Olaf, Teemo, Udyr, Vladamir