Avast but only with a well setup, I don't like the default settings in it.

Avast, free register and the best of it that it doesn't detect patches and cracks as harmful objects and the boot scan

Avast and AVG, both are quite good.

OS X user here, I don't have an AV running all the time. but I do occasionally load up Avast to do an on-demand scan. According to AV-TEST, it's the only decent free option on Mac.

I like Bitdefender Antivirus free edition - very light.

avast free is very reliable, i found it even more reliable then some of its paid versions

Avast, AVG and Avira

Microsoft Security Essentials.....fast, light, unobtrusive, and powerful enough to get the job done

Avira Free Antivirus

I would recommend Avira with Komodo Firewall and SpyBot search and destroy for mallaware.