I seem to be going against the grain here, but I use Norton AntiVirus.. my Mac is my main machine, but it is on a network with several PC's, and Norton for Mac also recognizes PC viruses. Since most of my downloading is done on the Mac, and then pushed off to PC's, I prefer getting rid of the viruses on OS X, where they are harmless, and not trusting the Windows machines (which also have Norton) to catch them all. Of course, if Norton on the Mac misses them, chances are really high that Norton on the PC will, too, but that's what tape backups are for.