I'm surprised no one has mentioned Microsoft Security Essentials. I know, I know, it's Microsoft blablablah and all but honestly, *knock wood* my system has been clean since I've been using it a year and a half ago. And yes, I was using Avast and Avira prior to my SE discovery.
I am with cracked McAfee for 3 years and I am very satisfied. I do not understand why you ask for "free" version? I use cracked antivirus, cracked win7, cracked office 2007, cracked photoshop, cracked dreamweaver, cracked CCleaner, cracked IM tools. BUT if you want the best FREE Antivirus then this is "Format C". I have doubled my hard disk, half is windows and programs, another are all my files. And remember no one virus will damage your computer until you give him access, therefor if you install some program and do not sure then use virtualbox or something similar.
Malwarebytes can only provides an additional automatic protection if you have the PRO version. The free does not provide a real-time protection that can supplement any antivirus installed. Although, malwarebytes offer a free trial for its PRO version, but after that, it won't offer additional automatic or real-time protection.
On the other hand, if you talk about manual scanning for malwares, malwarebytes is great and can detect majority of these malwares that antivirus cannot.