Don't buy into the claims that Mac computers can't get viruses. I learned the hard way when my entire system was wiped out.

Here's what I now have protecting my machine:

Virex features Universal Scan-At-Download to scan Zip archives, files compressed by StuffIt Deluxe, Compact Pro and more, as well as email attachment and drag-and-drop scanning. For OS X, version 10.0.3 and above.

Visit:

http://us.mcafee.com/root/catalog.asp?http://www.mcafee.com/products/virex/

Here's another good one:

RAV Anti-Virus

RAV AntiVirus for Mail Servers provides solutions for PostFix, CommuniGate Pro, DMail and Courier running on Mac OS X. Includes an anti-spam module.