Beer bottlecaps (or other drink bottle cap)
Preferably a glove (your hand may hurt after a while without one)
1) Take the bottlecap between your dominant hand's thumb and middle finger, with the top of the cap against the middle finger.
2) Hold your hand out forward, aiming the bottlecap edge away from you (your view of it should look like ||, not like O)
3) Snap your fingers!
If done properly, the bottlecap will dart out from between your fingers, launched away from you.
I'd post pics if my laptop would read my camera..I've been leaping around my apartment in dramatic poses wearing a glove with Roy's array on it launching bottlecaps around everywhere >D
EDIT: Gloves Strongly Reccommended. I cut my thumb a little bit without one :p