With all that said I still wouldn't run out and buy an expensive product to protect your Linux system, Linux is fairly protected due to it's really good security model and file system, but in theory an infection is always possible and what's even more possible is spreading Windows infections without even knowing it. It's a good idea to scan files in e-mail if forwarding or files downloaded from the Internet you plan to share - ignoring viruses altogether could hurt Linux in the corporate arena if it were considered "blind" to incoming threats to other platforms.   

On the darker side, as market share grows we may see a rise in threats to Linux, whether real or not, possibly created by those you pay to protect your system in the first place or by the competition to prove a point. That sounds more conspiracy theory, but what I have seen companies do to make money simply leaves me open to that possibility.  

I have evaluated six products on Kubuntu Linux running KDE and utilizing KMail, but any Ubuntu/Debian based distro should translate well to my evaluations. Note: Thunderbird does not have wizards in place for mail scanning so manual setup is your only option. Some RPM based distributions are more at risk as the system can be ran as root easily. That should be avoided as much as possible, especially when surfing the internet!

Results are based on ease of installation, interface, ease of use, effectiveness, mail support, ease of configuration, and ease of removal. You may wish to read up more on a product before making a decision and I wouldn't use my recommendations for production systems where a more complete product and evaluation may be required. These products are fine for home multiple user environments.  Here are my recommendations of the top six products.

My conclusion is F-prot and Avast are the best commercial "freeware" anti-virus products for Linux. Ubuntu/Kubuntu have included F-prot installer in the multiverse repos, which tries to pull down the engine, but both have RPM and DEB packages available for easy installation. F-prot is easily setup in KMail and Evolution for mail scanning from the wizards, which made F-prot a little nicer, but Avast won in every category other than mail scanning setup.

ClamAV is a perfect open source solution for mail and system scanning. It can have some configuration problems, especially in the updater piece, but once running it is solid! KMail and Evolution have wizards making mail scanning setup a breeze; although, mail scanning can be slow. KlamAV is not an anti-virus program, it is the GUI front end for ClamAV. Installing KlamAV or ClamTK along with ClamAV make it an easy all around open anti-virus solution.

Panda is perfect for scanning remote Windows machines from your Linux machine and maybe a good second defense on your Windows network if you are ultra paranoid about such things.

So what would I use? None. I am very cautious and have only had one virus in my whole twenty some year career. It was a Windows 2000 box and I had clicked a file in Morpheus before looking at the extension. Yes, you know you're burnt when you click an MP3 file in P2P and no music plays. I manually cleaned up the mess in less than 30 minutes, no big deal. In Linux I feel no threat whatsoever and would rather waste drive space on useful software, but if I had a bunch of teenagers using my system I'd most likely have F-prot installed.