Using the Havok physics you can get 3D characters to fully interact with their environments. Collision detection with the walls is fully general (no limitation to flat surfaces, or convex shapes). The character can bump into, move and jump on any object in the scene. Try shifting boxes around on the see-saw bridge, or jump onto the water wheel and see what happens. This demo has lots of polygons in the landscape so will probably run pretty slowly on slower machines without 3D cards.
Controls:
ARROW KEYS: move forwards / backwards and spin left and right
"a": sidestep left
"d": sidestep right
SPACE: jump
SHIFT: run
LEFT MOUSE BUTTON: grab an object