ClamAV® is an open source (GPL) anti-virus engine used in a variety of situations including email scanning, web scanning, and end point security. It provides a number of utilities including a flexible and scalable multi-threaded daemon, a command line scanner and an advanced tool for automatic database updates.

Features:

Command-line scanner
Milter interface for sendmail
Advanced database updater with support for scripted updates and digital signatures
Virus database updated multiple times per day
Built-in support for all standard mail file formats
Built-in support for various archive formats, including Zip, RAR, Dmg, Tar, Gzip, Bzip2, OLE2, Cabinet, CHM, BinHex, SIS and others
Built-in support for ELF executables and Portable Executable files packed with UPX, FSG, Petite, NsPack, wwpack32, MEW, Upack and obfuscated with SUE, Y0da Cryptor and others
Built-in support for popular document formats including MS Office and MacOffice files, HTML, Flash, RTF and PDF

Requirements:
OS X 10.4+