Here is an OpenGL demo that shows a raycasting volume rendering accelerated by GPU. This demo uses voxel 3D datasets from computed tomography. The demo is based on the Linderdaum engine, an OpenGL 2/3 rendeing engine with industrial and scientific volume rendering capabilities.
You can download the demo HERE.
On my GeForce GTX 280 (ForceWare 186.15) + Core2Duo E8400, the demo runs at about 1200 FPS.