Sixes is by far the easiest drinking game to learn/play.
Basically, you line 6 cups up and number them left to right.
You roll a single die. Whatever number pops up, you look at the cup that the number corresponds to. If the cup is empty, you fill it up with as much beer/whatever (works best w/beer) as you want to. If there's something in the cup, you drink it (usually best to impose a time limit so nobody decides to be a ***** and drink half a beer in 5 minutes).
Pass the die and repeat.
Be sure to laugh whenever someone fills a cup all the way to the top and then has to drink it on their next roll.
If you want to play a strip version of this game, you say that someone loses an article of clothing every time they drink out of a cup they were the last to pour into. So if you were the last to put beer in cup 4 and you roll a four, off goes your shirt, etc.
It gets a little hard to remember who poured into what when you're drunk, but I've had multiple incidents where it should've been me losing a shirt and I convinced some chick that she should lose her bra instead.