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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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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Americas Army

 

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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Battle for Wesnoth

 

A fantasy, turn based strategy in where you try to control towns on the map to gain gold and then use the gold to build your army to kick the crap out of your enemies.
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Globulation 2

 

Globulation 2 is a RTS game that is similar to Warcraft, but it tries to minimize micro-management by automatically giving orders to your units. Workers automatically gather resources. If you want to attack, just put your attack flag near the enemy base.
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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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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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Pysol

 

This is the first time since Windows 95 that playing solitaire has been fun and exciting. With over 200 game variations, this should satisfy everyone. And yes, it does have freecell.
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XMoto

 

XMoto is a 2D motorcycle game where you need to master the game's physics in order to successfully beat each level in as little time as possible. The controls are very simple - you can only accelerate, brake, lean forward or backward, and turn around. Don't let that fool you because this is a very fun game.
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Frets on Fire

similar to Guitar Hero

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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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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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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Tremulous

 

Tremulous is a FPS built upon the GPL'd Quake 3 engine. Aliens who can crawl up walls take on Humans and thier guns. Players build structures and kill enemies to gain points to upgrade themselves into more powerful units.
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Scorched 3D

 

Scorched 3D is based on an old DOS game called Scorched Earth. This game reminds me of the classic QBasic game Gorillas. You aim your weapon, adjust the angle and power of the shot, and try to blow your enemies into smithereens. Internet and LAN based multiplayer.
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Celestia

 

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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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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Nexuiz

 

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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TripleA

 

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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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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Enemy Territory - Fortress

 

A free mod for a free game! This is a FPS mod for Enemy Territory that is much like the Team Fortress mod for Half-Life. You choose a class (sniper, medic, spy, engineer, etc.) and join a team. There are several game play modes like CTF, King of the Hill, and Assault.
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