Structure Synth is an application for creating 3D structures from a set of user specified rules. It is an attempt to make a 3D version of Context Free.
It is built in C++, OpenGL, and Qt 4.3. Builds are currently provided for Windows and Mac. It is possible to compile Structure Synth on Linux as well.
As of now this project is usable, but it is still beta quality (although it is quite stable, not all planned features are completed).
The progress and design considerations can be tracked at the Syntopia blog.