Do you mean the one where you hold the phone near a speaker and it identifies the song?
The service is called Shazam, you can use it by dialling 2580 and holding the phone near the speaker. Or for Nokia phones you can download a program called Shazam ID which does the same.
Some SonyEricsson Walkman phones have a cut-down version called TrackID built in.
There is also a program for smartphones called Midomi, where you sing or hum the song into the phone and it tries to identify it.