ClamAV
ClamAV is a virus scanner under the GPL. On its official website, it is stated that ClamAV can detect over 20,000 viruses, and it can also recognize many different data types (including a variety of compressed files). It is designed for file servers and mail servers that serve Windows-based clients. In addition to Windows malware, it also scans for the handful of Unix malware. The Unix version is a command-line based utility
ClamAV's virus database is based on suspected viruses sent in by the community at large. It is rumored that, because of this, ClamAV occasionally has problems with false positives. Also, the ClamAV project's virus codenames differ from other antivirus companies, mainly because ClamAV is often the first on the scene.
There is also a Windows port available, Clamwin. It was ported over to Windows using the cygwin compatibility layer. A GUI wrapper was also created. A notable feature is its ability to email an administrator if it finds a virus. (However, given the ability of Windows malware to kill antivirus programs, this may only give a false sense of security. On the other hand, Clamwin is a new player to the field, and virus writers haven't taken notice of it - yet.) Clamwin is still in beta and is a little buggy.