My favorite is the cup trick:

http://www.youtube.com/watch?v=CYBqIRM8GiY

I've heard that called the "Spanish waiter" trick and seen it done with full plates. It's cool.

one interesting thing is that this bisection of a toroid is eerily similar to leon chua's (http://en.wikipedia.org/wiki/Leon_Chua) model postulating the existence of the memristor. in it, capacitance, induction and resistance are at three cardinal directions around a toroid and he supposed that something must exist at the fourth.

And this isn't one of them!