iAntiVirus is a free app that will scan for Mac-specific viruses and malicious software, both on your hard-drive and during real-time Web browsing. You access iAntiVirus through a menu bar icon, and the app can scan your Mac for current infections, with either a Quick Scan, a Normal Scan (longer but still relatively quick), or a Custom Scan of specific areas. You can set iAntiVirus to protect your Mac in the background (with a relatively modest resource footprint), and you can also schedule both scans and app updates every week, every day, or every other day, to make sure that you're protecting your computer using the most current virus definitions. Some anti-virus applications will scan for PC-specific threats as well (to make sure you're not passing on malicious software to PC users you interact with), but iAntiVirus is able to maintain a lower profile on your system by focusing on Mac threats only.
Arguably, the best anti-virus software for the Mac is already on your machine: the Mac OS itself, with its secure, Unix-based architecture and relative obscurity compared to Windows. There has never been a major virus or malicious software outbreak for OS X, so any antivirus app is of questionable usefulness--but iAntiVirus can provide some free peace of mind for anyone worried about future threats.