The World's Rudest Hand Gestures.

One useful bit of information for Americans traveling abroad:

Thumbs up
Meaning: "Up yours"
Used in: Greece, Latin America, Middle East, Russia, Sardinia, Western Africa

Depending on where you are, a thumbs up could just be a sign of approval. But in some countries, this refers to an action in the nether regions, and is meant as an offense. Visitors should note that to add insult to injury, the thumb can be jerked upwards.

More information here.