The above link (Source: Mashable) is a cool all-encompassing list of the big apps for music stuff (sharing, discovering and organizing).
I’d like to add one with a more human element: Soundtracking.
Have you ever loved a song because of the experience associated with it? Well, now you can relive those memories without having to sing the song in your head. You can take a picture with your camera and then apply a song to that image. Then it will let you share it as a package. Think postcard with a song/memory.
The only downside is that the music attached to an image is iTunes preview; I expect an update in the near future that links with Spotify (thank the lord… er, Spotify… for releasing APIs).
Product Link: http://soundtracking.com/
App Store Link: http://itunes.apple.com/us/app/soundtracking/id414323798?mt=8