Armagetron is a multiplayer game in 3d that attempts to emulate and expand on the lightcycle sequence from the movie Tron. It's an old school arcade game slung into the 21st century. Highlights include a customizable playing arena, HUD, unique graphics, and AI bots. For the more advanced player there are new game modes and a wide variety of physics settings to tweak as well.

http://www.armagetronad.net

BZFlag is a free online multiplayer cross-platform open source 3D tank battle game. The name originates from "Battle Zone capture the Flag". It runs on Irix, Linux, *BSD, Windows, Mac OS X, and many other platforms. It's one of the most popular games ever on Silicon Graphics machines and continues to be developed and improved to this day. It's one of the most popular open source games ever.

http://www.bzflag.org

The free space simulation that lets you explore our universe in three dimensions. Celestia runs on

Windows, Linux, and Mac OS X.

http://www.shatters.net/celestia

Enigma is a puzzle game inspired by Oxyd on the Atari ST and Rock'n'Roll on the Amiga. The object of the game is to find uncover pairs of identically colored Oxyd stones. Simple? Yes. Easy? Certainly not! Hidden traps, vast mazes, laser beams, and, most of all, countless hairy puzzles usually block your direct way to the Oxyd stones …

http://www.nongnu.org/enigma

Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization. The game commences in prehistory and your mission is to lead your tribe from the stone age to the space age...

http://freeciv.wikia.com/wiki/Main_Page

Tilt the floor to roll a ball through an obstacle course before time runs out. Neverball is part puzzle game, part action game, and entirely a test of skill.
Also found here is Neverputt, a hot-seat multiplayer miniature golf game using the physics and graphics of Neverball.
Neverball and Neverputt are known to run under Linux, Win2K/XP, FreeBSD, and OSX. Hardware

accelerated OpenGL is required. A 500MHz processor is recommended.

http://icculus.org/neverball

OpenArena is an open-source content package for Quake III Arena licensed under the GPL, effectively creating a free stand-alone game. You do not need Quake III Arena to play this game.
Bots were added starting in 0.6.0. They will spout taunts and everything!
Concepts would be loose, so don't expect a remake of Klesk in his original Klesk form, or any direct

remakes of any map/weapon/model

http://openarena.ws

Scorched 3D is a game based loosely (or actually quite heavily now) on the classic DOS game Scorched
Earth "The Mother Of All Games". Scorched 3D adds amongst other new features a 3D island environment
and LAN and internet play. Scorched 3D is totally free and is available for both Microsoft Windows and
Unix (Linux, FreeBSD, Mac OS X, Solaris etc.) operating systems.

http://www.scorched3d.co.uk

Sokoban YASC - Yet Another Sokoban Clone - for Windows. A wealth of features, e.g., deadlock detection, reverse mode, and replay mode. Good import functions and highly configurable, e.g., skins. Tools: Editor, solver, optimizer, and generator.

http://sourceforge.net/projects/sokobanyasc

StepMania is a free dance and rhythm game featuring 3D graphics, dance pad support, and a step editor.

http://www.stepmania.com

SuperTux is a classic 2D jump'n run sidescroller game in a style similar to the original SuperMario games.
The Milestone1 release of SuperTux features 9 enemies, 26 playable levels, software and OpenGL
rendering modes, configurable joystick and keyboard input, new music and completly redone graphics. Together with a little story, we hope to provide a good gaming experince with this first Milestone release.

http://supertux.lethargik.org

The Battle for Wesnoth is a turn-based strategy game with a fantasy theme.
A magical lightning strike
Build up a great army, gradually turning raw recruits into hardened veterans. In later games, recall your toughest warriors and form a deadly host against whom none can stand! Choose units from a large pool of specialists, and hand-pick a force with the right strengths to fight well on different terrains against all
manner of opposition.

http://www.wesnoth.org

Its highly awaited sequel, TrackMania Nations Forever, is now available as a free download. TrackMania Nations Forever offers a new complete "Forever" version of the Stadium environment, a complete solo mode and 65 brand new, progressively difficult tracks. TrackMania Nations Forever will unite an even larger number of players thanks to its engaging multiplayer modes, innovative online functions and revolutionary interactivity between players.

http://www.trackmania.com

Zombies is a turn-based game where the only goal is to outlast and destroy the zombies before they do
the same to you. Can you save humanity from being overrun?

http://codenautics.com/zombies