ClamXav for Mac is an open-source virus checker. Available from the App Store and many download locations, ClamXav for Mac installs quickly. The app is free, but donations through Paypal are requested.
ClamXav for Mac has a simple interface that displays a set of basic scan options and a larger window to show infected or suspect files. With updated definitions downloaded automatically, ClamXav for Mac can monitor your Mac in real-time, or you can install and run it on an existing system. To scan specific files or folders, simply drag them onto the ClamXav for Mac icon (handy for USB devices, for example). Files that are picked up as suspect can be quarantined or deleted. You can use wildcards to specify allowed files and file types.
We tested ClamXav for Mac on identical MacBook Pro devices, one with ClamXav for Mac installed and the other running a well-known (and expensive) antivirus package. Both picked up all our seeded apps with virus or Trojan signatures embedded in them, although ClamXav for Mac did flag a few normal files that the commercial app missed. By marking them as safe, we ensured that they were ignored in future scans. The only issue we noticed with ClamXav for Mac is the update frequency of the definitions files, which sometimes lagged the commercial products by a few days, but this is not a major concern. All in all, ClamXav for Mac worked well, and since it costs nothing, it should be a major consideration for those without AV protection.