ICBM3D (Inter-Continental Ballistic Missiles, 3D) is a 3D game of defense. Like the original "Missile Command" and its clones, this game places you in control of Anti-ICBM weapons which you use to destroy an onslaught of missiles (and other nasties) which are dropping onto your nation. The game ends when your cities have all been destroyed. You only gain replacement cities by acheiving certain score thresholds during each attack. The differences between Missile Command and ICBM3D is that ICBM3D, as the name suggests, provides a 3D perspective. You take advantage of X-Window's 3-button mouse to control your firing sight in 3-dimensions and change your viewpoint.
Quake 2 is a commercial 3D action game from id software, the makers of Doom. It is a shooter with a first-person perspective, in Doom's tradition. The game comes with three display methods. Anybody can run it on a virtual console (using SVGAlib) or under X. If you've got a 3D accelerator board based on the Voodoo chipset from 3dfx Quake 2 can use it to improve the display quality.
Freeciv is a multiuser reimplementation for Unix/X of the famous Microprose game of Civilization. By default, the game is an improved Civ II, but this can be customized; modpacks for near-100% compatibility with Civ I and Civ II are included. Multiuser gameplay is real-time: in each turn, all human players move concurrently. The game is designed to remain fairly playable even on poor network connections. Freeciv can also be played on standalone machines, and its AI players are a good challenge for beginners. The source code comes with the server, two X clients, and non-X clients for MS Windows and Amiga. Freeciv is released under the GNU General Public License. It is maintained by an international team of coders and enthusiasts, and is easily one of the most fun and addictive network games out there.
Wine is an implementation of the Windows API on top of X and Unix. It does not require Microsoft Windows, but can use native Windows DLLs if they are available. It provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes.