Some nice massive multiplayer games :

http://www.fearcombat.org/
http://icculus.org/alienarena/rpa/
http://combatarms.nexon.net
http://game.worldoftanks.com

Im currenty playing fear (also starwars resistance but is not free), fear its a cool
game and its free. If someone want play pm. Lets make a gog team and play
all together.

Not really massive multiplayer, but yes, nice multiplayer games to play on a tight budget.

I'd like to add America's Army 3 and Alien Swarm (both on Steam), as well as Wolfenstein: Enemy Territory

Martial Empires is pretty good. If not the best free MMORPG you can get. Great graphics, solid content, lots of active players etc. Only down side is the character customization.

http://martialempires.gamigo.com/

I'm not sure how many players they have (probably very few), but the free MMO's I know of are:

Planeshift
Lunia
Runes of Magic
Flyff
PowerUp

Edit:
I almost forgot about Myst Online: Uru Live.

Not forgetting that Warhammer : Online is now free
[url=]http://www.warhammeronline.com/[/url]

Warhammer Online is not free. It has an endless free trial.
Also, your links lead to this thread.

Ok thanks for letting me know :)