tunA
From MIT via Gizmodo:
tunA is a mobile wireless application that allows users to share their music locally through handheld devices. Users can “tune in” to other nearby tunA music players and listen to what someone else is listening to. Developed on iPaqs and connected via 802.11b in ad-hoc mode, the application displays a list of people using tunA that are in range, gives access to their profile and playlist information, and enables synchronized peer-to-peer audio streaming.
Thanks to Dan for the link.