Dont accept gifts from strangers ...
Netragard CTO Adriel Desautels borrowed a technique straight out of a plot from Mission Impossible: He modified a popular, off-the-shelf computer mouse to include a flash drive and a powerful microcontroller that ran custom attack code that compromised whatever computer connected to it.
Update: take care if you insert a USB memory key from an unknown source ... it could be a keylogger with a wireless transmitter ! Link 3