Favorite Beer Pong House Rules
If you and your partner both hit cups, you get the balls back and shoot again.
If you bounce it off the table and into a cup, it counts for two. Your opponents pick the second cup. If you bounce it in on the last cup, your opponents don't get redemption. Opponents are allowed to play defense once the ball hits the table, so you only usually want to bounce it when your opponents are distracted, incompetent, or plastered.
NBA Jam Rules: If you (you personally, not your team) hit two shots in a row, you announce "I'm heating up." If you make your next shot, you announce "I'm on fire" and then get to keep shooting until you miss.
Also, we usually play with 6 cups to a side, no spread. It makes the games go faster and allows more people to play.
The Cap Game
Kegs come with a plastic cap that covers the outlet. If you're throwing a big party, keep the cap. Once the party gets going, drop the cap in someone's drink. The person who got capped has to finish their drink, but then they have the cap to use on someone else. This game kicks ass at big, loud, busy parties.