The Suffering, Area 51 Full Spectrum Warrior, and Rise and Fall free
Didn't know these games had gone freeware with add support. Curtosy of the US Air force. Not exactly MMOs, but still good fun(and free!).
Full Spectrum Warrior: http://files.filefront.com/Full+Spec.../fileinfo.html < still holds up okay. it was stunning when it came out for xbox in 2003. It's a very uniqe games. you won't have played anything like it. the way you control it is different from anything before. was made originally as a war simulator for the army.
Rise and Fall: http://files.filefront.com/RF+Civili.../fileinfo.html < This game was a dud when it came out in 06. It ain't really bad per se, and it has big armies and some interesting ideas. The hero units in this game, you can actually go and play for a third person perspective. it's like watching over the should of one man in a big RTS game. I really like the concept.
I don't know if this is old news, but I didn't see them listed, and I just wanted to show them.
I recommend the suffering; it's a very good game. The ads are not even that bad, they only show up on the side of the logo on the title screen. The suffering is a classic and it won't cost a cent.