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.
Heroes is similar to the "Tron" and "Nibbles" games of yore, but includes many graphical improvements and new game features including 12 original soundtracks, 94 levels (in 10 different tilesets), and five gaming modes including Quest (the classical Nibbles), or Kill'em all (squish the lemmings).
XPilot is a graphical multi-player maneuvering game with less-exciting graphics, but more speed, tactics, and insults. Some people also use it for UDP network analysis. Over the past eight years of playing and development, tons of features have been added to the game, from basic weapons like shotguns and missiles to more recent ones like phasing devices, invisibility, or hyperjump.
U-Hexen is a port of the popular 3D game, Hexen by Raven Software/Id Software, to the Simple Directmedia Layer library. It uses an SDL mixer for sound and music. Some new menus were added for easier configuration. It works on Linux, FreeBSD, OpenBSD, and Windows. On FreeBSD, the game can work on the console without X.
EQEMu EverQuest Server Emulation Project is a project designed to fully emulate the EverQuest servers for use with the EverQuest(tm) client. This allows users to use their EverQuest(tm) client to connect to "unofficial" servers or to run their own EverQuest(tm) server. It is currently being developed for multiple platforms.
Flight Navigation Planner lets you make flight plans based on known airports, navaids, fixes, or cities. You can use the sectional charts, wacs, or the vector/terrain planning charts. It calculates headings, winds, time, and fuel. It features Airways-based Auto-Routing, Climb and Descent calculations (a/c type based), Fuel Stop Planning, Auto-Route around MOAS and Restricted Airspace, Hi-Res Weather Radar Overlay, Viewing of current sectional, wac, and IFR charts, the ability to see a route over TFRs, detailed nexrad radar overlays over your routes, Terrain Profiles with cloud ceilings, and the ability to upload flight plans to GPS.
Njam is a full-featured cross-platform Pacman-like game that uses the SDL library. It features single and multiplayer modes (local or network), duel games where players compete against each other to get more points, cooperative games where players cooperate to finish as many levels as they can, and great music and sound effects. It includes customizable level skin, many different levels, and an integrated level editor.