There's this:
http://linuxberg.xs4all.nl/preview/251301.html
 
and this:
http://www.ravantivirus.com/pages/isp.php
 
The first one is the one I hear the most about. I don't use one. The second one is strictly for mailservers, I think, just to check them coming through.
 
You really don't need to worry so much yet about AV products for Linux as long as you follow common sense and don't go opening mail attachments as root. Although there are a couple of viruses (worms maybe?) that I know about that Linux is vulnerable to, they can't really do much harm unless you make it a habit to be reckless with your root account.

Check out:
clamav - http://clamav.elektrapro.com
amavisd-new - http://www.ijs.si/software/amavisd/
openantivirus.org - www.openantivirus.org
Bitdefender - http://www.bitdefender.com/bd/site/products.php?p_id=16