I think the Quake (3) Online beta  is still free.

I'm sure you've all seen the ridiculous sales on Steam today. 
Holiday sale from Dec. 20th to Jan. 2nd with daily deals!  
 
http://store.steampowered.com/

Well there's a huge F2P MMO market that's growing.  I've been playing Fist of Fu, this 2D side scroller RPG a lot that reminds me of SNES games as well as Divine Souls, this steampunk brawler that resembles Lineage. 

If you haven't tried it yet.. 
 
Barkley Shut Up And Jam Gaiden    http://www.acid-play.com/download/barkley-shut-and-jam-gaiden
  It's an RPG. It's really well written and is pretty damn fun too.  
 http://www.youtube.com/watch?v=1M0FLAXXvRU (Teaser)  
 You don't need to like Baseketball to enjoy it. 

Alright I'll post this
 
 http://source.bungie.org/index.php?title=Get_Marathon

 Marathon Trilogy.

For anyone that's interested, I've created a list of free games with useful external links.  Feel free to suggest any additions, but I won't be adding anything and everything.  I want it to be a list of stuff worth checking out.

Warzone 2100 is pretty fun :)  There's even a portable version (no install and can run from a USB stick)

Anyone else have more recommendations?

There is a mystery game called The Marionette which I believe is free somewhere online. I need to go back to finish it.

I would look into sites like GoG.com for older games that are still great and also sold for cheap.

Lots of great indie games out there. Though judging by your taste you may want to try Section 8: prejudice. Looks fairly competent and fun in multiplayer. Cheap too

Tanks Online :! love that game.. play it every day.

League of Legends, but that probably has been mentioned before, great game!

Well, Team Fortress 2 now lol. That probably one of the best free games you will EVVUURR play.

Hydra Castle Labyrinth is a seriously awesome Metroidvania game, that has sweeeeet musak.

@S0ndor said:

@Chavtheworld said:

Hydra Castle Labyrinth is a seriously awesome Metroidvania game, that has sweeeeet musak.

Thanks for that link to a Japanese spyware website :/

Does it really have spyware? :(

Well, Team Fortress 2 now lol. The best free game you will EVVUURR play.

@S0ndor said:

@Chavtheworld said:

Hydra Castle Labyrinth is a seriously awesome Metroidvania game, that has sweeeeet musak.

Thanks for that link to a Japanese spyware website :/

Does it really have spyware? :(

@Barrock said:

@S0ndor said:

@Chavtheworld said:

Hydra Castle Labyrinth is a seriously awesome Metroidvania game, that has sweeeeet musak.

Thanks for that link to a Japanese spyware website :/

Does it really have spyware? :(

It's the link DeceasedCrab put in his Youtube videos, and I think I trust him more than I trust you Sondor.

@Chavtheworld said:

@Barrock said:

@S0ndor said:

@Chavtheworld said:

Hydra Castle Labyrinth is a seriously awesome Metroidvania game, that has sweeeeet musak.

Thanks for that link to a Japanese spyware website :/

Does it really have spyware? :(

It's the link DeceasedCrab put in his Youtube videos, and I think I trust him more than I trust you Sondor.

Maybe it's a misunderstanding on my part. It takes me to a Japanese website called vector that attempts to start an automatic download of an exe. Obviously I can't read kanji, but it looks like your typical faux anti-virus website. It looks like the kind of site you encounter when searching for free pawn.  Or maybe it's a legitimate download source for the game. I don't know. I can't read it, I don't trust it. That's my policy at least.

League of Legends...

Vindictus is a great online action MMO if you're looking for something different.  It's completely free with the option to purchase items at a cash shop. (not required, full access to all content is free)

Another Steam sale.

Valve are celebrating QuakeCon with a bunch of specials. There's a Quake and Doom Pack and a Quakecon pack, which also includes the Bethesda stuff ( Fallout, Oblivion ).

gang garrison 2 it's like TF2

I know it's 10$, but Limbo is a must have in the indie genre ! Overlord 2 was a really funny game too ! It was 2.50$ once or twice durjng sales in a pack with the first one in the series and its expansion. It should be like 5$ max now and it brings hours of fun !

MineCraft.. That's all...

I think there's a free version of Arma 2 out, worth a try in my opinion. Right now I'm addicted to Mount and Blade: Warband and minecraft, both good value. :) 
I picked up Limbo and From Dust, I love Limbo, and From Dust is fun, but I'd skip them both if you're tight for cash.

DEUS EX, ALWAYS ALWAYS DEUS EX

new code for 20% discount on greenmangaming 

Savage 2/League of Legends are your best bets imo.

10 % discount on direct2drive.co.uk using code  AFTERYOUSIR
 
I think it currently is the cheapest place to buy the digital version of BF3 Limited Edition for us Europeans. And no, I do not compare with keysites due to their dubious legality. 

10 % discount on direct2drive.co.uk using code  AFTERYOUSIR
 
I think it currently is the cheapest place to buy the digital version of BF3 Limited Edition for us Europeans. And no, I do not compare with keysites due to their dubious legality. 

Most european users will be able to get Football Manager 2012 at half the cost of the Steam price if bought through greenmanggaming. The game is steamworks activated so you will recieve a code by email and on your gamepage on greenmangaming before release.

For me the price ends on $34.16 (when using the code GOALS-GOALS-GOALS) compared to Steams price of 50€=69$

http://www.greenmangaming.com/games/simulation/football-manager-2012/

If you are not a member you can use the link below and receive an extra 5$ on your greenman account for your next purchase. Or get a link from a friend that already is a member.

http://www.greenmangaming.com/?gmgr=bexiwatu.

edit, updated 28/9

25% discount on all titles at greenman gaming when using code SUMME-RBLOW-OUT25

they have quite many steamworkstitles including deus ex

EA-games, including BF3, should be on site at latest next week

I must recommend:

1. Team Fortress 2

2. Minecraft

3 Age of Conan

Just a heads up, on The sequel of Monday night combat is currently in a "private beta" but it appears as they just take anyone who signs up on their website so its essentially just another free game.

Post release its supposed to continue being F2P i believe so there ya have it, its worth checking out atleast.

Thanks to @ZagZagovich for tipping me on this one

You should definitely play Achaea - quoted from the article here at GB

Features

• A unique, original, detailed mythology that is constantly shaping with new content.
• Player actions ultimately craft the universe, as behind-the-scenes writers update non-linear events.
• Absolutely free. Players may acquire any of the "cash shop" items through determination. Everything has its worth in gold coins.
• Thousands of unique combat abilities, allowing players to engage in memorable duels that are a test of true wit and strategy instead of hitpoints and levels.

Super Monday Night Combat just came out a couple days ago. It's free and on Steam.

Gog are currently having sale on EA strategy games: http://www.gog.com/promo/ea_worlds

Dungeon Keeper 1&2 for $3 each would be a must buy imo.