ClamXav for Mac OS X
ClamXav is a free virus checker for Mac OS X. It uses the tried, tested and very popular ClamAV open source antivirus engine as a back end.

McAfee Virex
Virex guards against all types of viruses and malicious code—including new and unknown threats—using the familiar Mac OS X user interface.

Intego VirusBarrier X4
Intego VirusBarrier X4 protects your computer from viruses by constantly examining all the files that your computer opens and writes, as well as watching for suspicious activity that may be the sign of viruses acting on applications or other files. With Intego VirusBarrier X4 on your computer, you can rest assured that your Macintosh has the best protection available against viruses of all kinds.

Sophos Anti-Virus for Mac OS X
Sophos Anti-Virus provides integrated cross-platform virus detection on Intel-based and PowerPC-based Macintosh servers, desktops and laptops. Our powerful Sophos virus detection engine scans all potential entry points for potential threats, and also detects non-Mac viruses that could be harbored on Mac computers.