ClamAV can run on Linux and it's free,
http://www.clamav.net/, but not sure if it has real-time scanning.
eTrust Intergrated Threat Management r8 is nice and documentation says has version for Linux, but I've never tried it on Linux myself on Linux,
http://www3.ca.com/Solutions/Product.aspx?ID=5704
Google search for Linux Antivirus came back with lots of hits, Panda, BitDefender, etc.