VirusBarrier
Although I do agree with the majority here that anti-virus software isn't really necessary on Mac, I use VirusBarrier X5. I got it as part of a MacUpdate Bundle a couple years ago. It runs smoothly and powerfully in the background. I don't even know its there unless the update window pops up. I have found a virus with this before and even used it to scan a roommates PC through the network. Found the problem and eliminated it. my 2 cents.
PS - VirusBarrier X6 is out and has way more features. ALSO ClamXav, which I used to use, is FREE. It just takes a little more manual labor.