This was a great tip from the folks at Zunerama

With one accessory - the Zune FM transmitter - you can stream any music you have on your player to nearby Zunes.

Most people who have the FM transmitter bought it for use in the car. The transmitter sends music on your Zune to your car?s FM stereo tuner, using available FM frequencies.

But, given that Zunes have built-in FM tuners, you can use this accessory for another useful function: playing DJ for nearby Zunes. Each Zune can tune in to the frequency being transmitted by one Zune with the FM transmitter.

Just another way to share music - and one that you can use for any music on your player.