I've been working on a IMGUI (immediate mode GUI) for Brigade 2 based on SDL, which is working pretty well and is quite responsive when the framerate is more than 5 fps. There are sliders for specularity, transparency and RGB values of the diffuse color of the material. Every material in the scene can be picked and updates in real-time + you can navigate the scene, control the vehicle and let a physics simulation run, all simultaneously while chaning materials. I will upload an executable demo and some videos of the GUI in action tomorrow.
Screenshot of the GUI while tweaking a glossy Ogre (almost 50k triangles, model from Javor Kalojanov's real-time raytraced animation demo):
szczyglo74 (CG artist/architect) also provided me with a modern half-open architectural structure (Altana model from Januszowka) which is great to test indirect lighting and which I will turn into a demo soon (screen rendered on 8600M GT):