The Prepared Piano from German artist Hauschka is quite a nice bit of piano music. Melodic and then dissonant in equal parts, it works as background music and still rewards close listening. From Hauschka's official site:

Hauschka is the alias of Dusseldorf?based pianist / composer Volker Bertelmann, whose work is based upon an exploration of the possibilities of the 'prepared' piano - a playfully disruptive intervention into the preconceived idea of the piano as a pure-toned, perfected instrument waiting for a gifted virtuoso to play on it.

Worth a listen, regardless of whether your taste in ambient tends towards the electronic or the "natural."