ClamAV

From Gripe Wiki

vinopasde ClamAV is an open source project, released under the GPL and hosted on SourceForge, that provides a free antivirus program for multiple platforms. ClamAV ports are available for many different versions of Linux, Windows, and Mac OSX.

From the ClamAV About page:

Clam AntiVirus is a GPL anti-virus toolkit for UNIX. The main purpose of this software is the integration with mail servers (attachment scanning). The package provides a flexible and scalable multi-threaded daemon, a command line scanner, and a tool for automatic updating