Everyone knows that exists tons of free games on the web, I'm not interested to list any crap that exists, please, suggest only good free games and mods, I will check out and update this thread as fast as possible.
For pratical reasons, I will not list limited games, with any region restriction, in beta phase or unallowed games modifications. Enjoy!
Action / Adventure / Battle / Platform
Abuse: website
ACE Online [1]: website
Armagetron Advanced: website
Audition [1][5]: ws (eu) - ws (ph) - ws (sg) - ws (uk) - ws (us)
Beneath a Steel Sky [1]: website
Between [9][10]: website
BZFlag: website
Canabalt [2]: website
Codename Gordon [12]: steam://install/92
Dance Online [1]: website
Death Rally: website
Disciple [1][2]: website
Facewound: website
Frets on Fire: website
Grand Theft Auto [1]: website
Grand Theft Auto 2 [1]: website
GunBound [1]: website
Head Over Heels: website
Little Fighter [4]: - [url=http://home.netfront.net/~marti/lf2/lf1.exe]download
Little Fighter 2: - [url=http://www.brothersoft.com/download-little-fighter-2-65984.html]download
Lure of the Temptress [1]: website
MDDClone: website
Mightier [9]: website
Narbacular Drop: website
Pirate Galaxy [1][2]: ws (de) - ws (fr) - ws (uk)
Rumble Fighter [1]: ws (eu) - ws (us)
Sam & Max: Episode 4: download
Samorost 1 [2]: website
Scarygirl [2]: website
Secret Maryo Chronicles: website
Soldat: website
Spelunky: - [url=http://www.derekyu.com/games/spelunky_0_99_9B.zip]download
StepMania: website
Teenagent [1]: website
The Goonies: website
Toribash: website
Urban Dead [1][2]: website
Wild Metal [1]: website
Casual / Family / Puzzle
La Brute [2]: ws (es) - ws (fr) - ws (us)
Peggle Extreme [12]: storefront
Peggle World of Warcraft Edition: website
Supaplex [4]: website
In-game web browser publishers
Armor Games [2]: website
Blurst [2]: website
Box10.com [2]: website
InstantAction [2]: website
Kongregate [2]: website
Miniclip Games [2]: website
Newgrounds [2]: website
Orisinal [2]: website
Rock Solid Arcade [2]: website
MMORPG / Roleplaying
4story [1]: website
9Dragons [1]: website
Anarchy Online [1][6]: website
Archlord [1]: website
Atlantica Online [1]: website
BattleForge [9][10][11]: website
Cabal Online [1][5]: ws (global) - ws (br) - ws (eu) - ws (us)
Dungeons & Dragons Online [1]: website
Ether Saga [1]: website
Eudemons [1][5]: ws (br) - ws (us)
Flyff [1][5]: ws (br) - ws (us)
Free Realms [1][6]: website
Furcadia [1]: website
Jade Dynasty [1]: website
MapleStory [1][5]: ws (global) - ws (br) - ws (eu) - ws (us)
Metin 2 [1][5]: ws (br) - ws (ca) - ws (pt) - ws (us)
Mu Online [1][5]: ws (global) - ws (ch) - ws (jp) - ws (kr) - ws (ph) - ws (tw) - ws (us) - ws (vn)
Pardus [1][2]: website
Perfect World [1]: website
Puzzle Pirates [1]: website
Savage 2 - A Tortured Soul [1][6]: website
Second Life [1]: website
Shaiya [1]: website
Silkroad Online [1]: website
RuneScape [1][2]: website
The Elder Scrolls: Arena [4]: - [url=http://www.elderscrolls.com/tenth_anniv/Arena106%20Setup.pdf]installation guide
The Elder Scrolls II: Daggerfall [4]: - [url=http://static.bethsoft.com/downloads/games/daggerfall_legal_and_installation.pdf]installation guide
Tibia [1]: website
Twelve Sky [1]: website
Twelve Sky 2 [1]: website
Wonderland Online [1]: website (th) - website (us)
Mods
Counter-Strike 1.3 (requires ): [url=http://www.filefront.com/Counter+Strike+13+steam/;2665833;;/fileinfo.html]download
Neotokyo (requires Source SDK): - [url=http://www.moddb.com/mods/neotokyo/downloads/neotokyo-client-07032009]download
Portal - Prelude (requires ): [url=http://www.portalprelude.com/]website
The Haunted (requires Unreal Tournament 3): website
Wing Commander: Standoff (requires Wing Commander - Secret Ops): website
Wing Commander: Unknown Enemy (requires Wing Commander - Secret Ops): website
Racing / Simulators / Sports
Fantasy Tennis [1][5]: ws (eu) - ws (jp)
FlyGear: website
GeneRally: website
Out of the Park Baseball 8: website
PangYa [1][5]: ws (jp) - ws (kr) - ws (th) - ws (tw) - ws (us)
Project Torque [1]: website
Racer - The Real Deal: website
The Hunter [1][6]: website
TrackMania Nations Forever: website
Upshift StrikeRacer [1]: website
Shooter
Alien Arena: - [url=http://download.cnet.com/Alien-Arena-2009/3000-7441_4-10447379.html]download
America's Army 3: website
AssaultCube: website
Battlefield Heroes [1]: website
Combat Arms [1][5]: ws (eu) - ws (us)
Cross Fire [1]: website
Cube: - [url=http://sourceforge.net/projects/cube/]download
Cube 2: Sauerbraten: website
Exteel [1]: website
F.E.A.R. Combat [1]:website
Flight Commander: website
Full Spectrum Warrior [3]: download
Gang Garrison 2: website
GunZ The Duel [1][5]: ws (br) - ws (us)
Landmass [1][5]: ws (br) - ws (ch)
Nexuiz: website
Quake Live [1][2]: website
Return to Castle Wolfenstein - Enemy Territory: - [url=http://download.cnet.com/Return-to-Castle-Wolfenstein-Enemy-Territory-full-install/3000-7441_4-10243511.html]mirror
S4 League [1][5]: ws (eu) - ws (kr)
Soldier Front [1]: website
Starsiege: Tribes: mirror
Starsiege: Tribes 2: - [url=http://www.tribesnext.com/files/TribesNext_rc2a.exe]multiplayer patch
The Babylon Project: website
Tremulous: website
Urban Terror: website
Voxelstein 3D: website
War Rock [1][5]: ws (kr) - ws (us)
Warmonger: website
Warsow: website
Wing Commander Saga Prologue: - [url=http://www.gamershell.com/news_35446.html]download
Wing Commander: Secret Ops [8]: website
Wolf Team [1]: website
Strategy / Environment simulators
Battle for Wesnoth: website
Bos Wars: website
Command & Conquer - Gold: - [url=http://largedownloads.ea.com/pub/misc/CnCGold.zip]download
Command & Conquer - Red Alert: download disc 1 - download disc 2
Freeciv [4]: website
FreeOrion: website
Glest: website
Ground Control [1]: - [url=http://www.fileplanet.com/betas/groundcontrol.aspx]game key
Navy Field [1]: website
Oolite: website
OpenCity: website
PiArmada (Vega Strike [7]): website
Rise and Fall: Civilizations at War [3]: - [url=http://www.gamershell.com/download_33916.shtml]mirror
Savage - The Battle For Newerth: website
Scorched 3D: website
S.W.I.N.E.: - [url=http://downloads.gamezone.com/demos/d14494.htm]mirror
UFO: Alien Invasion: website
Vega Strike: website
Warzone 2100: website
Wing Commander Privateer: Gemini Gold (Vega Strike [7]): website
== notes ==
[1] account creation or registration required
[2] in-game web browser
[3] ad supported version
[4] can require or [url=http://www.dosbox.com]DOSBox to run properly
[5] localized distribution, some publishers can apply restriction
[6] in-game limitations contested
[7] conversion built from another free game
[8] abandonware, can not run on recent operational systems
[9] mistakenly listed as a demo on storefront
[10] mistakenly named as a demo on "My games" list
[11] offered by Play4Free license / micropayment
[12] requires Steam client installed[/I]
== Last updates ==
2009.09.06 Added Shaiya, BZFlag, Racer - The Real Deal, Canabalt, and converted many of Steam free games on standalone links
2009.09.06 Added Oolite, FreeOrion, MDDClone, Head Over Heels
2009.09.20 Added Wolf Team, Glest, Warmonger, Bos Wars, Dungeons & Dragons Online, Tremulous, Voxelstein 3D and The Haunted
2009.09.20 Removed Dungeon Runners (will be discontinued)
2009.10.24 Lots of games added, queued and comming soon lists updated
== Author notes ==
I will only list really free or Play4Free (micropayment) games or mods
I will not list games announced like free but with limitation ingame
I will not list games with region restriction
I will not list games in beta phase
...and I will actively update this thread XD
== Not listed games ==
Refused by region restriction: MLB Dugout Heroes [1], Area 51 [3], Psi-Ops - The Mindgate Conspiracy [3], Prince of Persia - Sands of Time [3]
Coming soon: Dragonica, Blitz1941, Mechwarrior 4, Black Mesa Source, Danger from the Deep
Queued: Ferrari Virtual Race, Megaten, Ocular Ink, Volvo the Game, Syrth, Liero, Liero Xtreme, Gusanos, Wurmz!, OpenLieroX

Well, I have too many to list so I just point to my own site where I have such list available. Just go to links. Civilization, Infinity Engine, Ultima, Elder Scrolls, UFO / X-COM and Elite categories list official info sites, mods and modding sites and fan remakes/clones.

Holy cow.
Blocksum is a badass brickwell game. I don't really like most brickwell games except the original Tetris, but this one is great.
[url=www.runesofmagic.com/]Runes of Magic[/url] - a surprisingly polished F2P (cash shop) MMO. Similar to WoW in gameplay - which, regardless of what you think of WoW, is still pretty impressive by F2P standards. The importance of the cash shop is open to debate, but you mentioned micropayment-based games, and this is one.
Deadly Rooms of Death: Architect's Edition - an enhanced remake of the original DROD, a lengthy and very difficult environmental puzzler. Available on both Windows and Linux.

HL1 mods: Sven co-op
HL2 mods: Insurgency- tactical multiplayer game. more tactical then counter strike.
Synergy- Like Sven co-op
golden eye source
Minerva: metastasis

Sumotori Dreams
http://www.gravitysensation.com/sumotori/

For the Racing Games category: Trackmania Nations Forever

Someone posted this a few weeks ago:
Red Baron II:
Main Game Download
Super Patch Download

Yep, it was me.

Fanmade adventures:
Zak McKracken 2 (there is an english page, but I believe there is actually only a german version)
Broken Sword 2.5
Worms clones:
Hedgewars
Wormux
Chess:
Arena Chess GUI (needs a chess engine, some free one are linked on the page)
Jose (Another GUI, some engines are allready installed)
Arasan
Settler clone:
Widelands
Poker:
PokerTH
Kick-Off 2 clone:
Throw In
Colonization clone:
Freecol

Nice post mate :)
Hopefully you'll be better at updating that I am with the Demo's list :p
Karl

EPIC post.

Woah, extremely well-organized and useful post! Thanks!
EDIT
And I'll make a contribution by pointing out In the Wake of Gods, a free mod for Heroes of Might and Magic 3 with a ridiculously large amount of content.

A couple more:
Pox Nora - Free to play, pay to win (microtransaction based) multiplayer card battle/RPG game
Stendahl - Java-based 2D MMORPG
The Mana World - another 2d MMORPG
EgoBoo - Zelda-like action RPG game
SoulFu - Secret Of Ultimate Legendary Fantasy Unleashed, from the original creator of EgoBoo, basically a cell-shaded nethack
Parallel Realities - not really a game, but an indie developer of free games like Blob Wars: Metal Blob Solid, Project: Starfighter and The Legend of Edgar
OpenCity - city sim game
Daimonin - Free to play, pay to win (microtransaction based) isometric MMORPG
Thousand Parsec - Turn-based 4X game... hope you have a lot of time on your hands, this game can continue forever.
Flight Gear - Flight simulator... 'nuff said.
20,000 Light Years Into Space - Steampunk RTS
I think that'll do for now.

[url=sc2.sourceforge.net]The Ur-Quan Masters[/url]
A classic PC game whose full name everyone would surely recognize if not for a pesky trademark. It is now free.

wow, lot of suggestions, will update soon ^^