Windows only: Panda USB Vaccine disables autorun on your PC to protect you from being infected, but also protects your USB drives from spreading viruses when you use them in another PC.
Using the tool is a simple matter of installing and then launching it, and clicking the Vaccinate computer button to disable autorun completely on your PC, which prevents malware from spreading to your PC through a flash drive. The interesting feature, however, is the ability to "vaccinate" your flash drive, which essentially just creates a hidden, protected autorun file on the drive so that viruses won't be able to easily infect it — though readers should note it's probably not fool-proof.
Panda USB Vaccine is a free download for Windows only. Readers should note that Windows 7 has removed the ability to auto-execute files from a USB flash drive, more or less preventing this problem from happening in the first place.