TripleA is a clone of the popular board game "Axis & Allies", which is a turn based strategy wargame. TripleA also acts as a game engine allowing creation of custom games with similar mechanics to Axis and Allies. It supports online play, network play, play-by-email, and single player vs AI or hot-seat. It comes with 11 different maps and the option to download more from online repositories. Most games take place during World War II, though there are mods to play other conflicts like the Napoleonic wars, or Lord of the Rings.
Arx Libertatis is a cross-platform, open source port of Arx Fatalis, a 2002 first-person role-playing game developed by Arkane Studios. Arx Fatalis features crafting, melee and ranged combat, and a unique casting system where the player draws runes in real time to effect the desired spell. The Arx Libertatis source code is based on the publicly released Arx Fatalis sources. This does however not include the game data, so you need to obtain a copy of the original Arx Fatalis or its demo.
QTads is a cross-platform multimedia interpreter for Tads (Text Adventure Development System) games. Both Tads versions in use today (Tads 2 and Tads 3) are supported. It should be easy to port to a range of platforms, including embedded devices. MIDI, Ogg Vorbis, MP3, and WAV sound formats are supported.
Fabula is a Python game engine suitable for adventure, role-playing, and strategy games and digital interactive storytelling. It offers a client-server architecture, an event-based protocol, multiplayer and networking capabilities, a multi-threaded server, an abstract, visualization-agnostic model of a game world, an asset manager, and a plugin system. Gameplay can be recorded and played back. The code is well documented and runs on Linux and MS Windows (it should work on OS X, too, but this has not yet been tested). Fabula can be used as a library to develop your own games. As an alternative, you can use the included Pygame-based graphical editor and the default game engine.
The Brick Engine is a fast and simple lo-fi gaming engine. The engine provides a comprehensive API, including dazzling visual features, song playback, and collision detection: everything you need to make games with quickness and ease. The engine provides a straightforward C API and has language bindings for Tcl and Pascal, with support for more languages on the way.
FrobTADS is a complete rewrite of the Unix console-version of TADS ("Text Adventure Development System"). It uses curses (or ncurses) and provides an interpreter to play games developed with TADS as well as the TADS 2 and 3 development tools. It adds support for a number of relatively recent user interface features including full support for text and background colors, TADS 3 banner windows, and timed input. It's also much more portable and more easily maintainable, as it's built to modern Unix standards.