The "Rigid Bodies" demo is especially fun. From the description: "The motion of the rigid bodies is calculated by solving Newton's equation of motion and Euler's equations of rotation in discrete time." Whoa. I believe it is. The demo does real-time environment mapping and shadows, too.
Of course, if you have an NVIDIA card (or even if you don't), you'll want to check out NVIDIA's 3D demo page, as well. Lots of good stuff there, including a recently-added bump mapping demo for the GeForce. (I guess my G400 review lit a fire under 'em. Heheh.)