I'm downloading the Shattered Galaxy client right this moment, and oh jesus, it looks tasty. While I'm here, instead of opening a new thread I'll take this opportunity to ask - does anyone here play Populous : The Beginning online? I just got back on there after a year or more now I've sorted out my PC..I'm not very good, even when I used to play it every day, I didn't play for more than a couple of months before I had to leave it alone, but I'd welcome a challenge. Let me know.

Try Second Life.

No combat, but the ENTIRE world was solely built by players, you can also make real money (without having to give any cash away) by selling your creations and etc. online.

You need to pay to get property (to have a house and such) but otherwise the game is limitless, besides you get to make money (you can get it either by PayPal or your credit card).

F.E.A.R. Combat has been released recently, which is the free multiplayer variant of F.E.A.R, and is very enjoyable.

Furthermore, for MMORPG's I suggest you check out:

Eternal Lands - very small client (26 MB), but a very advanced 3d game.
MU Online - bit of diablo-like mmorpg, hack'n'slash

For first person shooters:

Tremulous - a refreshing game. Aliens vs Humans multiplayer game with RTS elements. (based on Quake 3 engine so should work on your pc...)
Nexuiz - unreal tournament clone, but with exceptionally good graphics (works on older pc as well with all options off)
Legends - free Tribes 2 clone. Seems to be very buggy though.

@chainsoar

I'm in a team who creates a 'matchmaker' for populous, which enables you to sit in a lobby, host/join a game and chat with other populous players. There are alot of mappacks available that you can play as well. And leagues enable you to compete with other players. There are also some clans active. So if you look for someone to play... see it here

And in case you're looking for a more childish (but not too much) MMORPG, get flyff.gpotato.com.

<div class='quotetop'>QUOTE(Koen @ Sep 17 2006, 10:30 AM) [snapback]255538[/snapback]</div>You created Matchmaker?

EDIT: I was thinking of the Matchmaker available from strategyplanet. I see now you've made your own version. Pretty cool!

Yu-Gi-Oh! PoW Joey The Passion. Its good multiplayer game, but not free....

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There is a currently is a beta that will be free upon release called Warrock, in myn opnion this game is one of the most exciting and realistic massive Multi player to date. You have the ability to pilot tanks, Humvees, Fighter jets (and to shoot your buddy out of the sky with a stinger LOL)boats and on and on... The graphics are very well done and the game play flows well. The game is another creation from the MMOG capital of the world South Korea. For more info and to download the client go to http://warrock.net

<div class='quotetop'>QUOTE(chainsoar @ Sep 10 2006, 05:23 PM) [snapback]254203[/snapback]</div>Ahh, I see you like Soldat ??? I found free game like Soldat, but much more better. (260 MB). It's absolutelly free, and only online. There is also level system (I'm at 23 lv., coz I played only 3 days :D). You can battle against other player or just solve mission ( 1-3 players at same time.) http://gunster.ijji.com its free like soldat game...

ActionCube (http://action.cubers.net/index.html)