Free online multiplayer games software for linux

Regnum Online is a MMORPG game, that wants to invite you to join our world and play for free. The only thing we require is that you fight for your realm.
Regnum Online is a massive multiplayer online role playing game (MMORPG MMORPG) for the PC where the player are immersed in a 3D virtual world. There they can live as Elves, Humans Dwarves, or Dark Elves having exciting adventures, exploring the wild lands or joining an army of players to take on the other kingdoms armies."
The game is in beta stage, but already offers many quests, items, and monsters, not to mention the large number of both english-, spanish-, and portuguese-speaking players youll meet in-game.
Main features:
- Regnum offers 3 realms to choose from.
- 9 fully customizable player characters.
- Hundreds of quests.
- The ability to capture forts and castles.
- Expansive lands to explore.
- Glorious graphical environments.

NetPanzer project is an online multiplayer tactical warfare game designed for FAST ACTION combat. Gameplay concentrates on the core -- no resource management is needed.
The game is based on quick tactical action and unit management in real-time. Battles progress quickly and constantly as destroyed players respawn with a set of new units. Players can join or leave multiplayer games at any time.
Official netPanzer development got basically halted in 1999, since 2002 netPanzer is available under the GNU GPL and thus Free Software and a GNU/Linux port of netPanzer is ready.
Main features:
- netPanzer is a game designed to fit into the real-time tactical game genre. Its art is based on realistic environments and vehicles. Battle will heat up as players challenge for territory, and without the need for resource management battles will emerge swiftly.
Network:
- Features a scalable client/server network architecture.
- Support from 1 to 100 players over the Internet or on LAN systems depending on the network connection type and game configuration (single/multi dedicated server vs. server-player, max number of units per player and so forth).
- Players will be able to join or leave multiplayer games at any time.
Gameplay:
- Real-time action without the need to gather resources.
- Manually aim your units fire while they are moving to another location.
- Maps range in size from 10x13 to 64x85 640x480 size screens.
- Three modes of multiplayer combat; objective capture, frag limit, and time limit.
- Change screen resolutions on the fly to 640x480, 800x600 or 1024x768
- A customizable interface layout.
- MiniMap for quick viewing of the world.
Special Effects:
- Particle system explosions, muzzle shots, missile trails, etc.
- Dynamic, real-time lighting.
- Translucent particles and shadows.
Enhancements:
- Change default masterserver to netpanzer.dyndns.org
- Fix remote DOS when sending invalid frame number
- Fix gcc 4.1 compilation
- Make server handle clients more gracefully that drop during connect

Wolfenstein: Enemy Territory is a Multiplayer Online First Person Shooter. Wolfenstein: Enemy Territory is a team game; you will win or fall along with your comrades. The only way to complete the objectives that lead to victory is by cooperation, with each player covering their teammates and using their class special abilities in concert with the others.

Featuring multiplayer support for as many as 64 players, Wolfenstein: Enemy Territory challenges gamers to the ultimate test of teamwork and strategy. Each of the five character classes is critical to a teams ultimate victory or defeat on the battlefield. The Covert Operative class allows players to steal uniforms, perform reconnaissance and gain access to enemy positions. While, the Engineer allows the Axis and Allied teams to lay and diffuse mines as well as build battlefield bridges, towers, forward command bases and other improvements in the midst of combat to gain advantages for their team.

Wolfenstein: Enemy Territory further online players the option to slug it out in the intense Team Last-Man-Standing game mode, where squad-mates cooperate to ensure their team has the last surviving man on the battlefield. With new game modes, character classes, weaponry, and added tactical skills, Wolfenstein: Enemy Territory will keep gamers in the trenches for hours.

Wolfenstein: Enemy Territory is a Multiplayer Online First Person Shooter. Wolfenstein: Enemy Territory is a team game; you will win or fall along with your comrades. The only way to complete the objectives that lead to victory is by cooperation, with each player covering their teammates and using their class special abilities in concert with the others.

Featuring multiplayer support for as many as 64 players, Wolfenstein: Enemy Territory challenges gamers to the ultimate test of teamwork and strategy. Each of the five character classes is critical to a teams ultimate victory or defeat on the battlefield. The Covert Operative class allows players to steal uniforms, perform reconnaissance and gain access to enemy positions. While, the Engineer allows the Axis and Allied teams to lay and diffuse mines as well as build battlefield bridges, towers, forward command bases and other improvements in the midst of combat to gain advantages for their team.

Wolfenstein: Enemy Territory further online players the option to slug it out in the intense Team Last-Man-Standing game mode, where squad-mates cooperate to ensure their team has the last surviving man on the battlefield. With new game modes, character classes, weaponry, and added tactical skills, Wolfenstein: Enemy Territory will keep gamers in the trenches for hours.

Reality Cyanide project is a client for Cyanide, a graphical virtual reality multiplayer role-playing game.

Cyanide is intended to be the most dynamic multi-player online role-playing game ever made.

This client uses an isometric perspective to present scenes. It is programmed with Python, pygame (SDL), and OpenGL.

Full 3D movement is possible, and the servers code can be exchanged online.

Wolfenstein: Enemy Territory is a Multiplayer Online First Person Shooter. Wolfenstein: Enemy Territory is a team game; you will win or fall along with your comrades. The only way to complete the objectives that lead to victory is by cooperation, with each player covering their teammates and using their class special abilities in concert with the others.

Featuring multiplayer support for as many as 64 players, Wolfenstein: Enemy Territory challenges gamers to the ultimate test of teamwork and strategy. Each of the five character classes is critical to a teams ultimate victory or defeat on the battlefield. The Covert Operative class allows players to steal uniforms, perform reconnaissance and gain access to enemy positions. While, the Engineer allows the Axis and Allied teams to lay and diffuse mines as well as build battlefield bridges, towers, forward command bases and other improvements in the midst of combat to gain advantages for their team.

Wolfenstein: Enemy Territory further online players the option to slug it out in the intense Team Last-Man-Standing game mode, where squad-mates cooperate to ensure their team has the last surviving man on the battlefield. With new game modes, character classes, weaponry, and added tactical skills, Wolfenstein: Enemy Territory will keep gamers in the trenches for hours.