Americas Army

somewhat replaces Counter Strike

A FPS developed by the United States Army (hence the name). I think it is probably the coolest recruiting tool that the army has ever developed. It is multi-player with single-player training missions, slow paced compared to Quake-style games, and very realistic (one shot, one kill happens often). Play this game, especially if you're an American, because you paid for it with taxes.
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Knight Online

 

Knight Online is a MMORPG that is fully 3D and has outstanding graphics for a freeware game. You can form a party with other players and fight monsters to level your character or complete quests. You can choose to be either an Orc or Human, and they occasionally have wars between the two species.
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Warsow

 

Warsow is a FPS built upon a modified Quake 2 engine that uses cellshading combined with fast paced action and acrobatics (trick jumps, dodging, etc) to create a hell of a game.
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Google Earth

 

Google Earth takes satellite imagery and combines it with the ability to tilt, zoom, search, create driving directions, measure distances, and show some buildings in 3D with textures to create a realistic 3D model of the earth. They also allow you to load in sophisticated overlays like terrain, roads, borders, airports, etc.
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Enemy Territory

 

A FPS that was going to be an add-on to Return to Castle Wolfenstein but was released as a standalone freeware game. Join either the Axis or Allies, choose your profession, and work with your team to capture enemy objectives. It is multi-player only, fast paced, team based, and loads of fun. ***Because it's OK to kill Nazis.***
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Legends - The Game

similar to Starsiege - Tribes

A free open source game extremely similar to Starsiege - Tribes. It has beautiful 3D graphics and supports online play with 30+ players in a match. The engine used in this game is called the Torque Game Engine and is created by some of the original authors of Tribes.
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Celestia

mostly replaces Starry Night

Celestia is a space simulation program. You can zoom in on planets and asteroids to see incredibly accurate detail and zoom out to see the entire galaxy. Until space travel becomes a reality, this is the best way to explore.
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World Wind

 

World Wind is an open source program created by Nasa. You can view the Earth in 3D from different satellite images combined with topographical data. I have always wanted to be able to look from space and zoom in all the way to my house.
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Alien Arena 2007

 

Yet another multiplayer FPS based off of the Quake 2 / Quake 3 GPL'ed source code. Alien Arena 2007 has many graphical effects and features include AI bots, alternate fire modes for weapons, a grappling hook, a built-in server browser, game mutators like vampire and regen, and multiple game types including CTF and Deathball. The game has a sort of 50's era sci-fi look, but it plays just like every other Quake based shooter out there.
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Dink Smallwood

 

This is probably one of the funniest adventure games I have ever played. It is an adventure game where you play the sarcastic hero. They used to sell this game but now they give it away as freeware.
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Nexuiz

somewhat replaces Quake

Nexuiz is a 3D FPS based off of the Quake 1 source code. They have added realtime lighting and shadowing, new particle effects, and an advanced menu system, among other things. It currently has several different game modes: deathmatch, CTF, Teamplay, Runematch, and Domination.
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Grand Theft Auto

 

The game that put Rockstar on the map and spawned several sequels. You are a gangster who steals cars and kills people for points. As you can tell, not much has changed from the newer games (which is a good thing). This game sparked a lot of controversy, but it one of the funniest games out there.
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Cube

 

A FPS that is similar to Quake. It features OpenGL, single/multiplayer, a physics engine, lighting, built in map editor, etc. The map editor can even have multiple people in a multiplayer game editing the map at once.
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Warzone 2100

 

Warzone 2100 is a real-time strategy game released by Eidos Interactive under the open source GPL license in 2004. It has nice pseudo-3D graphics, a multiplayer mode, and is quite fun to play. The game is similar to Earth2150.
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TripleA

replaces Axis and Allies

TripleA is an open source clone of the Axis and Allies board game. It is written in Java and has fully functional AI. I have been watching this game for quite a while and I am very impressed with the progress the developers have made. I am finally able to enjoy a game that I remember fondly from my childhood.
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SWINE

 

SWINE is an RTS game where the pigs and rabbits fight it out in the World War 2 era. Good graphics and level design make this game well worth the download.
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XaoS

 

Not a game but an incredibly fun waste of time. It is a real-time fractal viewer. What does that mean? It means you can explore colorful mathematical equations with interactive zooming, color cycling, a 3D filter, and more. Trippy.
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Starsiege - Tribes

 

This online FPS was huge back in the day. Now it has dated graphics, but it still fun to play. This game really kicked off the online capture the flag syndrome that almost every other online FPS has imitated ever since.
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Frets on Fire

 

Frets on Fire is a Guitar Hero style game where you hold your keyboard like a guitar and play. The game has great graphics that feel highly polished like a commercial game.
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Regnum Online

similar to Guild Wars

Regnum Online is a free MMORPG that is developed in Argentina. The world is divided into three separate realms (kingdoms) that are at war with each other. The game has both PvE and PvP, many quests, and 3D graphics that are on par with many other commercial MMORPGs.
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Dungeon Runners

 

Dungeon Runners is a free to play, fast paced point and click MMORPG with premium content from NCSoft, the makers of Guild Wars. They have done a fairly good job of putting humor into the game, which shows up in the NPC dialog and even the names of items (such as cardboard sword). The graphics and movement of the game remind me of World of Warcraft but the combat is much more like Diablo.
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Warmonger

 

Warmonger - Operation: Downtown Destruction is a freeware FPS that uses the Unreal 3 engine. It was created specifically for the Aegia PhysX physics processing unit. I wouldn't recommend buying one of these cards (at least not yet), but you can still play this game without a PhysX card if you computer is fast enough. It played as fast as a slideshow on my computer, so I really can't tell you anything useful about the game that you can't find on the official website.
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