Virex is recommended and available to .Mac members, and it seems to do pretty good.
I run absolutely no virus software and haven't done so since OS 9 came out. OS 7 was the most vulnerable to virii in my experience, as that was about the time that Apple had a little more market share and there were Macintosh-specific virii about.
I have yet to see or hear about any virii for Mac OS X. There have been some proofs-of-concept, but nothing came of them, and Apple fixed the holes before anyone could do anything destructive with them, if anyone was trying to do anything destructive with them.