Clam AntiVirus is an open source (GPL) anti-virus toolkit for UNIX, designed especially for e-mail scanning on mail gateways. It provides a number of utilities including a flexible and scalable multi-threaded daemon, a command line scanner and advanced tool for automatic database updates. The core of the package is an anti-virus engine available in a form of shared library.

Here is a list of the main features:

• command-line scanner
• fast, multi-threaded daemon with support for on-access scanning
• milter interface for sendmail
• advanced database updater with support for scripted updates and digital signatures
• virus scanner C library
• on-access scanning (Linux® and FreeBSD®)
• virus database updated multiple times per day (see home page for total number of signatures)
• built-in support for various archive formats, including Zip, RAR, Tar, Gzip, Bzip2, OLE2, Cabinet, CHM, BinHex, SIS and others
• built-in support for almost all mail file formats
• built-in support for ELF executables and Portable Executable files compressed 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, RTF and PDF

For more details:
http://www.clamav.net