I found out about VirusTotal today.
It's run by a Spanish company, and offers free, online virus checking.
The best part in my mind? It's crowd-sourcing your anti-virus.
You submit a suspect file, it's scanned by 42 different anti-virus applications, and the results get displayed to you. If the file is picked up by at least one of the 42 anti-virus programs, then they each get a copy of the file to test to improve their products.
By you testing a file, you're potentially helping keep everyone safe.