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ClamXav is a free virus scanner for Mac OS X. It uses the very popular ClamAV open source antivirus engine as a back end and has the ability to detect both Windows and Mac threats.

ClamXav can be setup up as passive or active: scan only the files you tell it to or your entire hard drive, whichever you prefer; you can also choose to activate Sentry to monitor your hard drive and scan new files as they arrive.

Introducing ClamXav 2

• Clean, simple-to-use interface
• Clearly lists infected files
• Free virus definitions (usually updated daily)
• Save your favourite scan locations for easy access
• Customise the toolbar - or hide it entirely
• Send selected files to quarantine or trash with one click
• Specify files to exclude using plain text* or by using drag & drop from the Finder
• Specify schedules to update virus definitions and perform scans
• Use ClamXav Sentry to monitor your hard drive and scan new files as they arrive†
• Use the bundled ClamAV engine or bring your own installation†
• Compatible with Mac OS X 10.5, 10.6 and 10.7 Lion

source: http://www.clamxav.com/