ioquake3 (or ioq3 for short) builds upon id Software's Quake 3 source code release. It cleans it up, fixes bugs, and adds features. Its goal is to be the Quake 3 distribution upon which people base their games and projects. Its design goals include optimizing the engine for playing Quake 3: Arena, Team Arena, and all popular mods. This distribution of the engine has been ported to many new platforms. While it doesn't have PunkBuster (and never will), it does provide more security for servers and clients by way of various bugfixes which aren't in id's client.
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.
SDLbits is a very lightweight SDL wrapper for Java. This library was designed to be very small and simple. It does not use Swig, but goes directly from "native" Java definitions to the sdlbits C wrapper library to SDL or OpenGL (in C). All symbols are exposed as they would be in C, except the "SDL_" prefix has been removed, since all names are inside the SDL class. The SDL structures from C are translated to very simple Java classes, such as SDL.VideoInfo. Most fields are exposed as read-only "get" methods, but "set" methods for writable members are supported as well. These classes simply interface to the actual C data with a ByteBuffer "pointer".
Octaspire Crates! is a scriptable, skinnable, extensible, and relocatable 3D action puzzle game. All the missions, game entities (or crates), game states, and configuration of the game engine are implemented as plain text Lua-scripts. So, if you know Lua, you can write new game entities, game states, missions, and levels with any text editor, without any special development tools. New skins can be created with any image editor that can save .png images, dropped into a new subdirectory under the resources/textures directory, and changed in the configuration file config.lua. Crates has also its own (simple) implementations for all the different container classes it needs (like vector, string, and so on) to keep the external dependencies fewer.
dsi is a simple game of invading aliens. You must stop those aliens from landing on you. It features smooth scrolling, high scores, 5 levels, simple controls, sound effects, and user selectable music. Working joystick support. Watch out for the tenacious mega! The graphics can be customized out of the box without requiring any recompiling. A simple definition file can be customized for coders.