0 A.D. is a free, real-time strategy game currently under development by Wildfire Games. It's cross-platform, 3D and historically-based. Wildfire Games is switching from a closed development process to open source, making the game's code available as GPL and the art content available as CC-BY-SA. People are encouraged to reuse and learn from and improve their work. They are not releasing a playable game, but want to show what they have achieved and make it easier for more people to get involved in bringing it closer to completion.

There's roughly 150K lines of C++ for the game engine, and another 25K for editing tools. Gameplay scripting uses JavaScript. The game is built on top of low-level libraries (OpenGL, OpenAL, ENet, ...), not an existing game engine (like OGRE).

They admit the code isn't all extremely clean or extremely modular or extremely well-written. It's been written by a wide variety of programmers, and it's far from a "completed" code base. They want to continue cleaning things up as part of the normal development process. They're not averse to rewriting entire subsystems if that's the most effective way. But in many areas it's quite solid.

Most of the code is specific to the game and not very useful outside it, but some parts could perhaps be interesting for developers working on other projects. Some examples:

• Low-level library providing cross-platform support for resource management (archive file loading, GL textures and shaders, ...), debugging tools (error reporting, memory tracking, stack dumps), timers, asynchronous IO, allocators, ...
• Converter from COLLADA models with skeletal animation into a simple game-friendly format.
• Binary XML caching system, for faster resource loading.
• Automated build tools using Amazon EC2.
• Boring maths code for matrices, vectors, quaternions, etc.

Check out their open source page for more details and info on how you can contribute.