ClamAV is a powerful AntiVirus scanner focused towards integration with mail servers for attachment scanning. It provides a flexible and scalable multi-threaded daemon, a command line scanner, and a tool for automatic updating via the Internet. Clam AntiVirus is based on a shared library distributed with the Clam AntiVirus package, which you can use with your own software. Most importantly, the virus database is kept up to date. The project was acquired by Sourcefire in 2007.
