Synchstep is a application in private beta that promises to play songs on your iPhone that are in tune with your stride. Developed by a guy over at digital agency Poke, Synchstep was originally planned to be a standalone MP3 player – but then iPhone came along (with the iTouch). The application promises that every step you take lands in-time with a drum hit, a bass pluck, a piano chord. Poke's blog gives some insight into the development of the app.
The idea behind SynchStep was to create an interface-less device. See, each person has a unique gait, and SynchStep is built to adapt to your individual walking style. It doesn't require people to wrap themselves around bizarre, non-ergonomic devices (keyboards? are you listening?).