Battle for Wesnoth: Polished turn based strategy game featuring fantasy characters and modular plots.
Cedega: Commercial re-implementation of the Windows API for Linux with a focus on gaming. Similar to (and a derivative of) WINE.
D2X-XL: Aims to create a better Descent 2 in OpenGL. Maps, levels, and sourcecode are available.
Defendguin: New Breed Software's clone of the Williams arcade classic "Defender." Instead of humanoids, you rescue penguinoids.
Gamma Patrol: Old-style vertical scrolling space-shooter. Fight your way through enemies using different weapons.
Gem Drop X: Fast-paced puzzle game where it's your job to clear the screen of gems before they squash you.
GTK Cervi: Clone of the game called Cervi (Worm). It is a multiplayer game (on one machine) where players drive worms and mustn`t collide with anything.
gtkmonop: GTK+ client to monopd. Monopd is a daemon for playing monopoly over the Internet.
Hatman: High resolution "Pacman" like game.
Lost Labyrinth: Modern graphical rogue-like game. Currently in a release state, it is also under constant revision. Source available.
NERO: Game and engine that revolve around "training" AI robots to engage on a futuristic field of battle.
OpenCity: Free 3d city simulator written from scratch using OpenGL and SDL.
Powermanga Linux: Arcade 2D shoot-em-up game with 41 levels and more than 200 sprites.
Vega Strike: Elite like space sim and game engine with modern rendering and capabilities.
WINE: Not directly a game, but a set of APIs that can be used to port Windows games to Linux or to run them directly.
WorldForge Project: Aims to produce an immersive environment engine for high quality games. It is both a community and a game engine with game implementations already finished.
XArchon: Play-alike of Archon which was created by FreeFall Associates and distributed by EA.
XFrisk: Networked X11R6 (Unix GUI) "Risk" like boardgame