If you have a pool table, or know someone who has one, you need to get one of these Wobbling 8 Balls.
They make wobbling cue balls as well, but I don't think that they are quite as effective, because the gag is exposed on the first hit. As the 8 ball is the last ball to be sunk in a rack, there's more build up.
I am lucky enough to work in an office that has a pool table, and there is always someone playing on it. One morning, I swapped out the standard 8 ball for this wobbling one. In a few minutes, I heard mumblings about someone having "broken" the ball. Whenever he tried to demonstrate how the ball was defective, however, it would roll perfectly straight—because we had swapped in the correct ball again.
This back-and-forth torment lasted nearly a half year. We stopped only out of pity.
Tip: if you hit this ball with a lot of force, it will go straight before the wobbling kicks in. This is because when a pool ball is hit, it slides across the felt without spinning until it slows enough for friction to kick in. The harder you hit the ball, the farther the spin-less slide. Knowing this, it's possible for you to not be affected by the wobbling effects, and, as a result, you can successfully pocket the ball.
Play this trick on someone you love before they play it on you!