This is a tank build I've been working on for Malphite, and it seems to work very good. In about 15 normal unranked 5v5 games, I've only lost one. ;)

This build is still WIP. Though, I still recommend it.

WIP

I'm not very good at choosing masteries, but since it's a tank it has alot (or everything) defense.

Warmog's Armor early in game makes you very tanky, and the sooner you get its bonuses, the better. Sunfire Cape is good in teamfights and pushing, might be exchanged though. The thornmail gives you very much armor, and also the return damage is effective.
Force of Nature is very good, and I do not recommend you to exchange it (but maybe buy it earlier). It gives you some needed magic resistance (you don't have very much of it now) and gives you an epic life regen, something about 20 HP/s.
At last, Randuin's Omen (however, might be good to buy it earlier, haven't tried it yet), extremly good in team fights.

Seismic Shard is good support (this build is far from OMA anyway) and can give you alot of assists.

Heal is a must since you're a tank. Personally, I prefer ignite. But it can be exchanged to Exhaust, Flash or Clarity.

The only tank build I can play D:
Please, comment if I should do some changes.

Remember, (always) go first in team fights.
I think this build is better in 5v5 than 3v3, but I haven't really tested.