Cave Story
Cave Story is highly regarded as an excellent free game. The visuals, music, and sound effects are highly reminiscent of NES games and old computer games. It's a side-scrolling platformer/shmup. Unlike the games from it's time that it resembles, it isn't terribly difficult, unless you play the special post-game area. In the link above, either download the Japanese version, then download the English patcher to the same folder and apply the patch, or download the deluxe package, or download the appropriate port.
Alien Swarm
This game is offered for free on Steam. It's an isometric shooter with strategic elements.
Marathon Trilogy
The Marathon games were developed by Bungie, and were the precursor to their widely popular Halo series. The Marathon games play a lot like the Doom series, but they feature some nice little touches and dual-wielding weapons. Go
here to download Aleph One, an engine for running the Marathon games. Go
here to download a version of Marathon 1 that's modified to work with Aleph One. Go
here for the other 2 Marathon games.
Command & Conquer
The first 3 Command & Conquer games are free. They're great real-time strategy games, along the same vein as Starcraft. All I know is Q said his favorite is Tiberian Sun, so check that one out.
Nanaca Crash!!
A fun, addictive little timewaster with a silly premise; you're a schoolgirl riding on her bike that plows into some poor guy, and you have to try to get that guy to fly as far as possible.
Scorched 3D
A 3D update to the classic Scorched Earth, it's a war game where you aim shots strategically to try and hit your opponent, similar to games like the Worms series and Gunbound.
Nexuiz
As advertised by it's website, "Simple, fast, intense, and completely free!" This first-person shooter features fast-paced gameplay and interesting weapons.
Quake Live
Quake Live is the smash hit Quake III Arena, only browser-based and free. A must for any FPS fan. It has a handy feature that helps gauge your skill level so you'll be matched up to people around your skill level.
Peggle Extreme
Available through
Steam for free, Peggle is an addictive puzzle game. You shoot a little ball and try to angle your shots to hit the proper dots.
Trackmania Nations Forever
Also available through Steam, this is a racing game with crazy physics.
America's Army 3
Available through Steam, this is a very realistic FPS developed by the American goverment as a recruitment tool for the army. Regardless, it's very good, and free.
I Wanna Be the Guy: The Movie: The Game
Taken from the website: "I Wanna Be The Guy: The Movie: The Game is a sardonic loveletter to the halcyon days of early American videogaming, packaged as a nail-rippingly difficult platform adventure. Players fill the role of The Kid, a youthful, vaguely Megaman-esque protagonist on a quest to become The Guy. This inscrutable plot, however, is just a vehicle for a wide variety of inventive, well-designed and frustrating jump-and-shoot challenges that pay homage to many of the games you loved as a child. The ever-fragile Kid explodes in a shower of red pixels at the slightest brush from the game's many obstacles, from traditional spikes and bottomless pits to more unconventional killers, such as plantlife and puzzle pieces.
Using a multiroute layout not unlike a Metroidvania, the game grants a degree of deadly exploration, without those extraneous upgades meant to make life easier. The game provides players with a choice in terms of their deathrate, thanks to a variable difficulty setting that changes the number of save points from frequent to nonexistent. IWBTG is open to all players; knowledge of videogaming history is optional, and may not help against the frequently ironic and always sadistic deathtraps located herein. And so, the question is left up to you..."
Kingdom of Loathing
A browser-based MMORPG with a purposefully lazy aesthetic design for amusement, this game has a bizarre sense of humor, from classes like Disco Bandits, Seal Clubbers, and Saucerors, to enemies like drunken half-orc hobos and giant goths.
Portal: The Flash Version
Normally, I'm not too keen on flash games, but this one replicates Portal's gameplay nicely in a 2D setting.
R-Type
You can download R-Type for free, and play it with
this Amiga emulator. It's no surprise that roms for emulators are free, but this one is a legal download, as it's offered by the makers of R-Type. Also on the same page is an R-Type clone they made, and some dinosaur game.
There are also a number of free mods for commercial games, most notably games like Half-Life/Half-Life 2, the Doom series, Oblivion, and Fallout 3.