Just seemed like a prudent thread to make considering how many topics have of people asking "hou to find teh free games". So I am going to make a list. Just post your favorite free games, links to them, their genre (throw in a brief description if you want) And I'll add them to this post.
To get things started here are two sites i know of that are very helpful for those looking for free entertainment:
http://play-free-online-games.com/Exactly as the name says, I have found many fun times wasters here.
http://www.freewarehome.com/It's where I tend to look first when I am looking for productivity software, never checked the games section but it has one.
Links from: "All of us"
http://www.onrpg.com/boards/907.htmlhttp://www.onrpg.com/boards/46858.htmlLinks from: Bind
http://www.caiman.us/Look here to find many more. Great freeware game site!
From: Wizard
http://gamelist.mmosite.com/search/game.shtml?f=Free===============================================================================
RPGs:Dink Smallwood:
http://en.wikipedia.org/wiki/Dink_SmallwoodJust search around and you'll find plenty of places to download it (freeware), personally I don't think the game play is that great but if nothing else it's really loving funny.
Survival/Horror:Survival CrCIA Z:
http://skasoftware.com/files/scz.exeFrantic zombie survival at it's best.

(Thanks to: NickTheSushi)
The Suffering:
http://news.bigdownload.com/2008/09/25/download-the-suffering-full-free-game/Play an inmate trapped in a prison filled with demons? Turn into a giant spider? Yes. May not be W7 compatible.
Action/Shooter:Chex QuestEssentially a Ultimate doom total conversion, I got the original disk out of a box of Chex (guess that promotion worked because I bought it just to get the game). Epic game, boot spoon and all.
http://www.chucktropolis.com/downloads/ChexQuest3.zip(Thanks to :ChexGuy331 &Bind)
Wolfenstein: enemy territory“It was suppost to be a expansion pack for Return to castle Wolfenstein but due to issues it was released for free as a stand alone game” –Zackin5
http://www.splashdamage.com/wolfet(Thanks to: Zackin5)
Urban Terror“I used to play Urban Terror, its a free CS-style shooter except with worse graphics :P
Used to be a q3 mod, now runs standalone. It's got plenty enough maps and gun to keep someone entertained for ages.
Although it is a pretty big download.” -Antallion
http://www.urbanterror.net/news.php(Thanks to: Antallion)
F.E.A.R. Combathttp://projectorigin.warnerbros.com/fearcombat/main
(Thanks to: zackin5)
Iji:
http://www.remar.se/daniel/games.phpPlay as a girl-become-cyborg as you blast whatever you want.
(Thanks to: Supermario566/Aenima)
Grand theft auto 1 & 2:
http://www.rockstargames.com/classicsSteal cars, blow up things, kill people, ect, ect.

(Thanks to: Madmitten & zackin5)
Wild Metal:
http://www.rockstargames.com/classicsDrive a car/tank thing and kill other car/tank things

(Thanks to: zackin5)
Cross Fire:
http://www.filebuzz.com/fileinfo/54698/Cross_Fire.htmlLike combat arms except I don't think the guns expire.

(Thanks to: Block Builder)
Oolite:
http://oolite.org/downloadAn Elite clone, also one of the few mac friendly games on the list so far.
The Ur-Quan Masters (Star Control 2):
http://sc2.sourceforge.net/downloads.phpA true classic.
Universal Combat:
http://files.filefront.com/Universal+Combat+Full+Free+Game/;9367664;/fileinfo.htmlFly a space ships, tanks, fighters, and even get out and walk.
Savage - Battle for NewerthCategory: RTS/Shooter
Put Medieval times, pew pew laser guns, and monsters in a blender and you get this.
Link:
http://files.filefront.com/Savage+The+Battle+For+Newerth+Free+Game/;5467503;/fileinfo.html
(Thanks to: BladeOfTheHorizon)
Mu-cadeCategory: Shooter
Fight off the Physics Centipede invasion.
Link:
http://www.asahi-net.or.jp/~cs8k-cyu/windows/mcd_e.html
(Thanks to: BladeOfTheHorizon)
NanozoaCategory: Shooter
Destroy dieseases such as the Swine Flu (Just kidding) in your microscopic spaceship thing.
Link:
http://browse.files.filefront.com/Nanozoa+Official+Demos/;1889544;/browsefiles.html
(Thanks to: BladeOfTheHorizon)
Typhoon 2001Category: Shooter
Another seizure-giving game.
Link:
http://typhoon.kuto.de/download.html
(Thanks to: BladeOfTheHorizon)
Duck Doom DeluxeCategory: Shooter
Who in the world uses a BFG9000 to kill ducks?
Link:
http://noproblo.dayjo.org/DDD/(Thanks to: BladeOfTheHorizon)
Gang Garrison 2:
http://www.ganggarrison.com/Pretty much a parody of Team fortress 2, Except 2D and 8-bit style
(Thanks to: Aenima)
RRootage:
http://www.pcworld.com/downloads/file/fid,64976-order,1-page,1/description.htmlGalaga-like shooter, With more bullets (Correction; ALOT more)
(Thanks to: Aenima)
Warning Forever:
http://www18.big.or.jp/~hikoza/Prod/index_e.htmlSame as RRootage, Except not as difficult.
(Thanks to: Aenima)
Fraxy:
http://mokeron.hp.infoseek.co.jp/zg/fraxy_main.htmlLike RR and WF, Except with a boss creator and Is easier than both RR and WF
(Thanks to: Aenima)
Cavestory:
http://www.cavestory.org/2D-Shooter

(Thanks to: Aenima)
Americas Army:
http://www.americasarmy.com/I am honestly up in the air as to whether this is a sim or not, for now, it's a shooter.

(Thanks to: tommy909808)
HarmotionCategory: Shooter
That game where you kill evil polygonal shapes.
Link:
http://files.filefront.com/Harmotion/;6352029;/fileinfo.html(Thanks to: BladeOfTheHorizon)
RayhoundCategory: Shooter
A game that tends to be fun. Also, to download it correctly, right click and save target once you get to the website.
Link:
http://www18.big.or.jp/~hikoza/Prod/dlcount.cgi?product=rh(Thanks to: BladeOfTheHorizon)
GunroarCategory: Shooter
This game gave me seizures.
Link:
http://www.asahi-net.or.jp/~cs8k-cyu/windows/gr_e.html(Thanks to: BladeOfTheHorizon)
Cholo:
Category: Shooter
A very old cyberpunk game that was made around the time of Pac-Man, and it looks like someone found an old copy of it and updated the graphics/gameplay.
Link:
http://download.ovinebydesign.com/cholo/download.aspx(Thanks to: BladeOfTheHorizon)
Prism: Guard ShieldCategory: Shooter
A typical shooter where everyone that plays the it online takes it way to seriously. Like the people that play Halo 3.
Link:
http://www.prismthegame.com/downloads.htm(Thanks to: BladeOfTheHorizon)
Stellar Frontier:
http://www.stardock.com/products/sf/download.htmlCan be played online and offline. It's a space combat game that used to cost money but now is free, problem is the online servers died. There are plenty of independent servers available for online play if you look for them (Just Google it).
Starsiege Tribes:
http://www.fileplanet.com/140246/140000/fileinfo/Starsiege:-Tribes-(Free-Game)
(Thanks to ChexGuy331) I actually paid for this game when it came out. That graphics are dated but it's still fun to shoot people and capture flags.
Tribes 2:
http://www.tribesnext.com/downloadsNow also free it seems.
Area 51:
http://www.bigdownload.com/games/area-51/pc/area-51-free-full-game/(Thanks to ChexGuy331) The PC version is free, which is a shame considering I have the actual arcade machine in my house. I don't know how the PC version plays but I can play through the arcade game with only the four quarters needed to get started, using both guns by myself.

RTS:Command & Conquer gold:
http://ccgold.ea.com/uk/The classic battle of GDI vs. NOD.

(Thanks to: zackin5)
Battle Forge:
http://www.brothersoft.com/battleforge-225477.html"genre-blurring online RTS" -gamesradar

(Thanks to: troy001234)
Glest:
http://glest.org/en/index.phpA medieval war game about magic vs. technology.
Command & Conquer Red Alert:
http://compactiongames.about.com/od/freegames/p/ccredalert_free.htm?once=true&The classic RTS about soviets and allies is now freeware. Let's all give thanks to promotional giveaways.

Battleships forever:
http://www.wyrdysm.com/games.phpSequel to Warning forever, Except its an RTS, And has a ship editor.
Ancient war:
http://www.caiman.us/scripts/fw/f3415.htmlTop-down Ancient RTS With level editor
(Thanks to: Aenima)
Ground Control:
http://www.fileplanet.com/156136/150000/fileinfo/Ground-Control-(Free-Game)
Never played it but I've heard good things.
Rise & Fall:
http://www.gamershell.com/download_33916.shtmlAge of empire styled RTS but with several unique features that make it well worth playing. Reviews I've seen range from "awesome" to "crap", go figure.
Sim:OrbiterCategory: Simulation
A very complicated space simulation.
Link:
http://orbit.medphys.ucl.ac.uk/download.html(Thanks to: BladeOfTheHorizon)
LinCityCategory: Simulation
A free version of SimCity.
Link:
http://lincity-ng.berlios.de/wiki/index.php/Download/Installation(Thanks to: BladeOfTheHorizon)
Full Spectrum Warrior:
http://files.filefront.com/Full+Spectrum+Warrior/;11928826;/fileinfo.htmlI haven't played it but It did bring in a number of awards for being a great infantry sim.
Fighting::
Toribash:
http://www.toribash.com/Fighting game, make you own moves, kick people's heads off.

(Thanks to: Zenthos)
MUGEN:
http://www.heroesofgaming.com/mugenA fighting game in which you download your own characters and stages. It's your customizable fighting game.
(Thanks to: Supermario566/Aenima)
Little Fighter 2:
http://www.lf2.net/I haven't had this much with a fighting games since the super smash brothers games.
SynaestheteCategory: Rhythm/Fighting
A game that will give you more seizures than Gunroar.
Link:
https://typo3.digipen.edu/index.php?id=988#download
(Thanks to: BladeOfTheHorizon)
Turn-based strategy:MegaMek:
http://sourceforge.net/project/showfiles.php?group_id=47079Set in the battletech universe, using what else of course: mechs.
Battle for Wesnoth:
http://www.wesnoth.orgBest free turn-based fantasy war game I've ever played. (also, only?)
FreeCiv:
Category: Strategy
This is one awesome freeware classic.
Link:
http://freeciv.wikia.com/index.php/Download
(Thanks to: BladeOfTheHorizon)
Misc:The Powder ToyThe Powder Toy is an awesome physics simulation game, similar to some Java games online (but better!).
http://powder.unaligned.org/(Thanks to: Bind)
Sam & Max: Abe Lincoln Must DIE!:
http://www.telltalegames.com/samandmax/lincolnmustdieSam and Max now must stop the president. Watch these two go on another wacky adventure!
(Thanks to: Supermario566/Aenima)
De Blob:
http://www.fun-motion.com/physics-games/de-blobLooks blobtastic.

(Thanks to: n0cturni)
Phun:http://www.phunland.com/wiki/Home
2D physics sandbox
(Thanks to: Aenima)
GL TronCategory: Miscellaneous
Again, another awesome freeware classic that was updated.
Link:
http://www.gltron.org/download.php(Thanks to: BladeOfTheHorizon)
Armagetron:
http://armagetronad.net/Like GLTRON, Except more Modifiable, And has online play.
TobloCategory: Miscellaneous
According to the sounds in the game, I'm pretty sure the whole game was voiced by 6 year-olds.
Link:
http://toblo.csnation.net/download.html(Thanks to: BladeOfTheHorizon)
Platformer:BOXWORLDhttp://box.totalwarp.co.uk/
(Thanks to: tails)
Eversion:
http://zarat.us/tra/offline-games/eversion.html"deceptively awesome" - VGCats

(Thanks to: Aenima/Shadowlink764)
Icy TowerCategory: Platformer
You climb an endless icy tower as Harold the Homeboy.
Link:
http://download.cnet.com/Icy-Tower/3000-2099_4-10075060.html?part=dl-IcyTower(Thanks to: BladeOfTheHorizon)
A Game with a Kitty.
Category: Platformer
Like Cave Story but the main character is replaced with a kitty.
Link:
http://www.origamihero.com/games.php#1
(Thanks to: BladeOfTheHorizon)
Knytt stories:
http://nifflas.ni2.se/index.php?main=02Knytt_Stories2D-platformer With level editor
(Thanks to: Aenima)
Mabinogi:
I don't know what it is! yay!
http://mabinogi.nexon.net/Downloads/Client.aspx(Thanks to: kakashi11)