It always fascinates me how different gestures mean different things in different countries. Here are a few to be aware of:
Next time you are in Brazil consider not using the thumb-touching-the-forefinger gesture. In New York it means "OK" however in Brazil it refers to an intimate part of the anatomy.
A few other international differences:
In England, that "V" sign with your palm facing inward is the equivalent of the middle-finger sign in the US.
Asking someone to walk over to you by crooking your finger at them can be an insult in Asia and the Middle East.
In Greece an upward nod of the head means "No," while tilting the head to one side means "Yes." If you raise an open hand to refuse something offered to you in Greece, your gesture may be understood as "go to hell." Greek wrath is then very clear.
In parts of Southeast Asia, standing with hands on hips as you address someone projects hostility. This is definitely one to avoid when facing an armed official.
Happy traveling and mind those gestures.