The FM Transmitter lets you wirelessly push your music to a nearby radio, providing connectivity with the most common audio device, present everywhere. It can be used to throw a party using your music collection on your phone, or to listen to your mp3s in your car.
I believe, it's one of the most user friendly features regarding music sharing. While UPnP is a common feature on the Nokia smartphones, UPnP enabled devices are not so common around the households of your friends, I guess :-)
By simply launching the FM transmitter via the Music Player, you are able to choose an unused frequency (it should be easier in Europe or Asia, than in the US) and start broadcasting your Nokia station (with RDS nonetheless). I have yet to meet with a phone (N78, N79 or N85) with this function, but it should be common across the portfolio :-)
To see, how it works, please check Symbian World's video on practical use of the FM transmitter:
