A fun game if you like music.
An app that plays some characteristic part of a song or music piece (the beginning few seconds, the bass line, etc.), and you have to identify it as quickly as possible. You can invite friends to a challenge. Leaderboards show the best overall, by music genre, decade, etc.
Discover new songs you might like.
Software that has a huge lyrics database and suggests new song based on the lyrics of the songs you like.
Many like to whistle familiar tunes.
A website where you can publish an audio or video clip of you whistling familiar tunes. Others can rate them, and the best get shown on the hit list page and promoted.