Sophos is offering it's full-featured anti-virus program for the Macintosh for FREE (for home and personal users).
You get business-grade protection for your Mac, from the same company that provides enterprise protection for my employer at work.
Did I mention it is FREE? Did I mention that Sophos is very reputable in the anti virus and security field, specializing in enterprise and education/government markets? Did I mention that there is no longer ANY excuse to leave your Mac unprotected?
This software is also very low overhead. It does not popup warnings every three seconds, it does not slow your Mac down, it DOES protect you against threats, and prevent you from passing on dormant threats via email to Windows users.
It works with Mac OS X, versions 10.4, 10.5, and 10.6.
No more excuses. Protect your Mac!
(BTW: If you do download and install this, I am interested in finding out via the comments if you found any viruses on your first scan.)