My favorite has always been the one where you say "I'm going to [insert made up trip or activity here] and I'm bringing/going to see/going to eat [something that starts with A]." And then the next person says the A word and adds a B word. And you go like that through the end of the alphabet.

It can be pretty tame, sure, but with the right friends, it's always uproarious and often X-rated. The one last night was definitely off-the-wall, and turned into a long crazy narrative about, uh, insertables.