New Version 8 now available! Even more full-featured with scheduled scanning, Live Anti-Virus and a new look and feel. If you're an existing user, you don't need to do anything—you'll be updated automatically.
Sophos Anti-Virus for Mac Home Edition
That's one free one, and of course there is the Free VirusBarrier (non realtime scanning) available in the Mac AppStore as well as the full blown VirusBarrier for what is it $69? And there is the ClamXav for free as well (also non realtime scanning.
I would have listed Symantec but I am seeing too many issues with Lion and Snow Leopard right now.