Gizmo's Freeware is an invaluable site for those of us looking for quality free software for Windows, Mac and others. As part of the site, they have several sections that are updated regularly with topics such as Best Free PC Games and Best Free Antivirus software - the latter of which just called out a new champ.
The new king of the free antivirus hill for now is Qihoo 360 Total Security, a product from China. Now, my personal mild paranoia would disincline me to use a security product from China, unless I had the ability to be rather sure it was not sporting any "backdoors", but that's just me.
The site also wisely offers this caveat:
To begin with let me say this: there is no best antivirus out there. Why do I say this? Any product that you take will behave differently against various virus samples since the AV engines and other components incorporated in them are of different technologies.
The rest of the tested products are here: