ClamAV

Introduction
Clam AntiVirus (ClamAV) is a free, cross-platform antivirus software tool-kit able to detect many types of malicious software, including viruses. One of its main uses is on mail servers as a server-side email virus scanner. The application was developed for Unix and has third party versions available for AIX, BSD, HP-UX, Linux, Mac OS X, OpenVMS, OSF (Tru64) and Solaris. As of version 0.96 ClamAV builds and runs on Microsoft Windows. Both ClamAV and its updates are made available free of charge.

http://en.wikipedia.org/wiki/Clam_AntiVirus

Gentoo
su - root emerge clamav

Ubuntu
sudo apt-get install clamav

https://help.ubuntu.com/community/ClamAV

Updating
sudo freshclam

Usage
clamscan /name/of/directory