With the recent trojan scare PC Tools' timing for the beta release of iAntiVirus for Mac could hardly be better. While there are a variety of anti-virus applications for the Mac, iAntiVirus seems to be especially designed to reduce resource usage by simply ignoring virus signatures for Windows. The idea is that your Mac is immune to Windows viruses so why waste memory, etc. scanning for them? Otherwise iAntiVirus is pretty conventional with a menubar interface and real-time scanning.
In some ways I'm of two minds about this approach. It's true that I don't allow any Windows boxes on my home network so having a Mac-only solution makes sense. However, by not scanning for non-Mac viruses it's possible that your Mac might unwittingly pass along a virus or trojan by email, etc. I run an Enterprise version of Sophos provided by my University and I've been surprised by how many Windows virus signatures it has picked up on my machine from various downloads.
iAntiVirus is a free download, but virus definitions and updates are $29.95 for one year.