AVG is complete fail. They are the only AV company I know of that doesn't receive an outcry when they release epic fail database and program updates. I mean come on, people still use them when at least TWICE a year they release an update that causes massive issues, in some cases requiring a format and re-install of windows???
Avast has gotten really good with their latest release, probably the lightest in terms of usage, especially with it's new scan caching.
AntiVir has the best detection of all the freebies, handily beats/ties the grandaddies like Kaspersky. It is very light on resources, but there is the nag screen (though you can block it).
Panda Cloud is nice, but it does require an internet connection to be effective.
MSE is nice and unobtrusive. But I have had some users machines get infected with it installed. Those users have now been switched to Avast and are chugging along nicely.
Rising AV is also decent, I have it on some test machines/test users. They have been infection free as well.
Comodo's freebie I've tried before. It always ends up randomly blocking emails if you use a program rather then web based emails. And you don't know it until you uninstall it and see some emails coming in from months ago and such. Maybe they fixed that issue now, haven't used it since that was discovered.