I can spend all day in this thread just posting stuff.
Ok, these may have already been posted and I missed them, so bear with me:
Mabinogi.
IMO, this is one of the only free MMO's to really compete with the commercial games. There's free games with better graphics, but this by far has the most dynamic gameplay. The amount of skills and things you can do are amazing.. You can even compose your own music, sit on a street corner, and get donations for entertaining people.
Atlantica Online
This is the other MMO which I believe can compete with the pay for ones.
This is unique, for being a turn based MMO. Think Final Fantasy Tactics, with a timer. You can eventually accumulate up to 8 mercs, and in PvP you join other groups for big 100 on 100 battles.. It can get a bit nuts. The best thing about this game, is the cash shop is totally optional.. You can experience all the content, without ever spending a dime.
Other games:
Survival Crisis Z
This used to be pay for, but now it's free. You're in a city filled with zombies, there's random safehouses populated by one of three factions, and you can take missions from any single faction. You can also simply take over houses by bombing them terrorist style breaking their resolve, or just beat them down the old fashioned way.
It's a pretty fun game, and if the graphics were better it'd compete with Dead Rising or Resident Evil series as the best zombie themed games out there.
Think GTA with zombies and no cars, and you'll get an idea how what this games about.