Well since there are no MacOs X viruses, hasn’t been one released since 2006, you don’t need an anti-virus application. A good program to check for tracking cookies etc. is called MacScan, it has a 30 day demo version. Most likely though you don’t need anything until someone releases a few dozen Mac viruses. The 7 Mac trojans out there require you to download and install them with your admin password. They are in fake or illegal software.

ClamXav is a free on-demand virus scanner for Mac, because it does not monitor, it will not slowdown your system at all.
The other guy is right, you don’t really need it, but it doesn’t hurt to have it either.
ClamXavhttp://www.clamxav.com/