I wanted to make a thread dedicated to quality, free gaming. My goal is to streamline your browsing experience if you are looking for something to play that's both free and fun. There's so much out there, so I hope people come in with their own recommendations (and links please) as well!

GOG's freebies
(See Elenarie's thread for updated installation packages!)
Steam's freebies
(I'll avoid listing any duplicates from these sites, but keep in mind that many of Steam's free-to-play games do not rely upon the Steam client, and can be accessed through developer websites instead)

- Abobo's Big Adventure
Type: Flash
Genre: Everything!
About: This game is the greatest tribute/love letter to the NES that you will ever see. It's a mash-up of many popular NES games, and features more character cameos than you are likely to recognize. It plays extremely well and is an obvious labor of love.

- Angry Birds
Type: Chrome application (requires Google Chrome web browser)
Genre: Catapult
About: Say what you want about this game's marketing saturation and Rovio's general dickishness, Angry Birds remains a rather enjoyable time waster. Now you can play it for free on PC.

- Battle for Wesnoth
Type: Download and install
Genre: Turn-based strategy
About: An open-source turn-based strategy game. Elements of randomness within a simple and focused design. Can be played both single- and multi-player.

- BEACON
Type: Download, no install
Genre: Short platformer
About: It's a game about a little astronaut man stranded on a distant planet. The game is extremely short, only taking about 20 minutes to complete. Great atmosphere and tight control.

- Burrito Bison
- Burrito Bison Revenge
Type: Flash
Genre: Catapult
About: This is my favorite catapult game. Launch a large, muscular bison-man-thing into fields of terrified gummy bears. Sports a cool upgrade system, too.

- Cave Story
Type: Download, no install. Use "English Translation Patch by Aeon Genesis" to patch the game text to English.
Genre: Metroidvania platformer
About: A sprawling one-man project. Considered by some to be the greatest indie game of all time. Getting the good ending is one of the most arduous yet rewarding tasks in all of gaming.

- Desktop Dungeons
Type: Download, no install
Genre: Roguelike puzzler
About: A paid version is out now, but this original free version is still a wonderful experience. Deceptively simple at first, this game can suck you in quickly.

- Desktop Tower Defense
Type: Flash
Genre: Tower Defense (duh)
About: There is an infinite number of Flash-based tower defense games, but DTD stands as the most widely recognized and admired.

- Dwarf Fortress
Type: Download, no install
Genre: Roguelike city-builder
About: One of the most robust and complex games ever created (and it continues to grow). Has an absurdly steep learning curve, but its most devoted fans will tell you that it's worth the dedication. Custom tilesets to replace the ASCII graphics do exist, if you can't stand the lack of graphics.

- Freeciv
Type: Download and install
Genre: Turn-based strategy and colony management
About: An open-source turn-based strategy game based largely on Civilization 2. Gives you the full civ experience for free. Can be played both single- and multi-player.

- Grand Theft Auto 1 & 2
Type: Download and install (one-time e-mail registration, installers are DRM-free)
Genre: Sandbox destruction
About: The grandaddy of 'em all. I think GTA 2 is the better game, but they are both worth checking out.
Protip: If you don't want to register your e-mail, use 10-minute mail for your download link. Once you get the installer, it's DRM-free so you can just back it up wherever you like.

- Knytt Stories
Type: Download, choice of install or zip
Genre: Metroidvania platformer
About: A remarkable platformer with absolutely sublime control. It also features some amazing music. Lots of user-created content available as well.

- League of Legends
- Heroes of Newerth
Type: Download and install
Genre: MOBA
About: Extremely popular competitive games mixing elements of PvP RPG and RTS. Excellent client and developer support. The community can be abrasive, though, so I suggest playing with friends if you can.

- Marathon Trilogy - Installer--Source Port
Type: Download and install (Installer and Source Port)
Genre: FPS
About: An old-school FPS trilogy from Bungie originally released on Macintosh computers. All three games were released as freeware by Bungie, and made playable on modern operating systems through the Aleph One source port.

- Mighty Jill Off
Type: Download, no install
Genre: Upward-moving platformer
About: You might recognize this gal as a hidden character in Super Meat Boy. This game is pretty fun in its own way, with vertical level design and a lesbian-slave playable character.

- Quake Live
Type: Browser-based plug-in
Genre: Competitive multiplayer FPS
About: It's essentially Quake 3 Arena running within your browser. Plays extremely well. The player base has dwindled since its debut, so jump in while it's still alive.

- Robot Unicorn Attack
- Robot Unicorn Attack: Heavy Metal
- Robot Unicorn Attack: Evolution
- CANABALT
Type: Flash
Genre: Run right to win
About: These are several what I like to call "run right to win" games, but the original RUA remains my favorite. It's also the gayest by a decent margin, in a strangely good way...

- RunMan: Race Around the World
Type: Download, no install
Genre: Speedy platformer
About You might recognize this guy as a hidden character in Super Meat Boy. This is his original game and it's a damn good one. Lots of levels and fast, Sonic-like gameplay.

- Spelunky
Type: Download, no install
Genre: Roguelike platformer
About: Derek Yu's solo masterpiece, Spelunky is a game that mixes Indiana Jones with Nethack. It's levels are both randomized and fully destructable. Use this beginner's guide if you need some help getting into it.

- Super Crate Box
Type: Download, no install
Genre: Arcade score attack and survival
About: A robust and tight arcade game with a wonderful retro presentation. I especially love the music tracks for the stages. Beware, as this game is tough as nails.

- Tetris
Type: Flash
Genre: Seriously? It's Tetris!
About: It's Tetris, for free, in your browser.

- TrackMania Nations Forever
Type: Download and install (Also free on Steam, though not on its 'Free to Play' list)
Genre: Arcade racing and track construction
About: Free version of Trackmania, with a full track set and building tools available gratis. Extremely tight control designed around keyboard use.

Here, this should help, although it seems it's not updated anymore :)

http://www.gog.com/en/forum/general/free_games_and_3rd_party_mods_the_updated_list/page1

Ah, guess he beat me to it, figured I searched thoroughly enough :-)

+1 for the info - thanks for the hard work :)

Well it seems to be out of date, so i guess you can copy his list, and maintain it. :)