This BoomBench from German designer Michael Schoner of NL Architects plays music from passing mobile phones via Bluetooth.  Via CribCandy:

"Urban Play is an international project by Droog Design, created and curated by Scott Burnham and presented as part of ExperimentaDesign Amsterdam 2008. For the design of the Boombench NL architects transformed a regular piece of street furniture into a sound system.

This seemingly normal street bench reveals its hidden gimmick when it is activated via Bluetooth, allowing the public to play the music on their mobile phones."