Thanko introduced the so-called i2!, a 2.4 GHz digital wireless transmitter for portable audio players.
After connecting your MP3-player to an i2!, you can share music with up to 5 people within a radius of 10m. The device features a frequency response of 10Hz-24,000Hz, a bit rate of 16bit at 48kbps Stereo and an audio latency of 20ms. 5 hours of music playback are possible after charging the i2! for 5 hours. (link)