Plane9 3D Music Visualizer
Plane9 is a free and commercial (4.95 euros) 3D visualizer. You can create your scene with an node-based editor. The scenes are saved in a readable format so you can also modify them with a text editor. Plane9 comes with more than 100 tutorial scenes and the rendering engine is based on OpenGL API.
More information and downloads: Plane9 homepage.