ClamAV is an open source (http://www.open-source.org) and free anti-virus program released under the GNU General Public License (GPL). The home page of the project is http://www.clamav.net/. ClamAV includes:
A command-line virus scanner
A client/server virus scanner
A tool to update the virus definition database
More than 60,000 signatures of viruses
The ClamAV runs on Windows (see ClamWin), Linux, BSD, Solaris, and Mac OS X. The list of supported platforms and the binaries are available at http://www.clamav.net/binary.html#pagestart.
ClamAV is primarily designed to scan email attachments. It can also be used as a regular anti-virus. It can decompress archives (e.g., erar, zip, gzip, bzip2, cab) and scan Linux mailboxes (mbox, MailDir, and raw emails), and it supports on-access scanning on Linux and FreeBSD.
There are several third-party projects derived from ClamAV. The list is found at http://clamav.net/3rdparty.html#pagestart. Most of these projects aim at integrating ClamAV with mail delivery tools and proxies, or at providing a graphical interface to the command-line tools.