BitTap

BitTap is a four-lane rhythm game, but it ships with no songs. Point it at the music already on your phone and it listens to each track, finds the beats, tempo and key, and charts a tap map for every difficulty.

All of the analysis happens on the device: the audio is decoded, run through onset detection and beat tracking, and turned into notes. Gameplay is drawn by a custom view locked to the audio clock, so taps stay in time. No network, no accounts.