I used the SFML 2.0 release candidate, and a precursor to Dan’s Live Audio Lab.
Cool thing number 1: the ground texture is produced with a cellular automaton.
Cool thing number 2: the ground is interpolated noise. To make jumping work right, so that when you jumped on a slope that went up-right you headed up-left, I had to differentiate the interpolation function. It’s not incredibly impressive; both interpolation and differentiation are common, but I can’t remember seeing them together.