Just like ping-pong, this can be played either in singles or doubles. Supplies: players, beer, and a ping-pong table (or make your own, described later).
Each player fills a cup with beer and places it one paddle-width from the end of the table, in the center (or a paddle-width from the side for doubles). Hitting your opponent's cup earns you a point and requires the opponent to sip (5 sips to a cup). If you get the ball in your opponent's cup, you are awarded 5 points and the opponent must drink whatever remains in the cup (excluding the ball).
No player may touch the ball prior to its hitting the table or a cup; if the ball hits a cup before hitting the table, it remains in play even after a single bounce on the table. A player may attempt to save a point after the ball hits a cup by returning it (provided of course the ball has bounced no more than one time). Multiple hits count only as a single point (exception: a ball hitting a cup and then bouncing inside the cup, or inside the partner's cup, counts as a 5-point inside-the-cup "poofter").
A number of house rules are common - knocking a cup over is grounds for a penalty chug (but no points); ball may not be "auto-returned" via a fortuitous bounce off the cup, etc. The trajectories of the volleys should be high arcs, to keep things civilized (and to maximize your chance at a poofter).
Travel hints: Beer pong nets can be created using some string, two chairs, and three or four paper towels.