Hold off on that McAfee patch!

McAfee AV Patch Leaves Some Computers Unbootable | Maximum PC

Oopsy-daisy! According to complaints on McAfee's message board, a mandatory service pack for the company's antivirus VSE 8.7 software has left some machines unbootable. The update, which was issued on May 27 and later pulled on June 2, was intended to squash minor security bugs, but also inadvertently flagged some Windows system files as malware.

This is another reason why I recommend NOT using one of the big commercial anti-virus programs.
I recommend using AVG Free.

Mac anti-virus programs

iAntiVirus - Free AntiVirus for Mac

There are more and more threats being created for Mac computers. iAntivirus is a free anti-virus program that may be suitable for most Mac users. It only targets Mac-specific threats (so, if you are not sharing files with a PC or dual-booting to Windows, then this should be sufficient).
The free version of iAntivirus comes is full-featured and only lacks the support features - updates are automatic and free.

One other drawback to this free software - it doesn't cover OS versions prior to 10.5

For other programs, check out these reviews at About.com It seems the most comprehensive anti-virus (in these reviews) is the paid Norton Internet Security for MacIntosh program.

If you have other recommendations, please leave them in the comments.