ClamAV is a free and open source antivirus solution, probably best suited towards advanced users.
The program has become popular among users of production software such as web and mail servers. ClamAV includes a number of powerful tools for servers such as multiple command line tools, on-demand scans and virus database updates. It can all be configured by editing the included configuration files.
Version 0.100.1 is a freeware program which does not have restrictions and it's free so it doesn't cost anything.
Virus and Malware Tests: The program has been tested with several antivirus applications and has tested clean of viruses, malware or trojans.