The problem with most drinking games is that there's a lot of having to manage what drink goes with what action. Really, if you're going to actually conduct one of these things, it's best to stick to one thing and treat it like a one-line bingo game.
The Washington Post's Sleuth column has a handydandy checklist of keywords and catchphrases that's pretty easy to follow. If John McCain decides to go ahead and attack Barack Obama on assorted individual issues, or vice versa, then you should be pretty much guaranteed a rollicking good Wednesday.
(For the sake of variety and in the interests of something resembling completion, here's The Daily Show's version, the Washington City Paper's, and a short one from the people boycotting Oprah's sponsors. Your guess is as good as mine.)
[Update: Of course, DebateDrink.com has a downloadable pdf version of their game. Crazy bastards. Anyone else I'm missing?]