The Moving Coin
Effect:

The magician just puts a coin on the neck of the bottle and without tuching the coin it keeps moving.

Secret:

1. Freeze tthe bottle (it must be a plastic bottle).
2. Dip the coin in water
3. Put the coin on the neck of the bottle

NOTE FROM ICHIGO: More of a science experiment than a magic trick, but still very fun to do.

I tried this with glass and plastic bottles, big and small. I liked the results I got from a 2-liter soda bottle. Make sure that the coin covers the entire opening of the bottle. That is VERY IMPORTANT. I used either a half-dollar or a plastic bottle cap.
The coin doesn't really move around, it sort of just stays in place and "flaps" up and down. You'll have to do it to see what I mean.

Also, in response to the comment below...Here's why it works:

When the bottle comes out of the freezer, the air inside the bottle begins to quickly warm up to room temperature. As the air warms up, it expands. Since there is no room for the "bigger" air to go, it pushes the coin out of the way!

The bigger the bottle, the better this will work - since there is more air inside a bigger bottle. For me, a 2-liter bottle works fine.