Tangerine

Tangerine is an amazing little Mac application from Potionfactory (http://www.potionfactory.com) that scans your music library and analyses all of your songs, determining their beat intensity and beats-per-minute value. You can then make playlists based on this data. What this means is you can make a playlist for slow relaxing music, hard workout music etc without having to hunt those song types down using the genre or artist field in iTunes.
I tend to listen to music by genre as I work - sometimes I feel like classical, or Pink Floyd, or Dire Straits, or movie soundtracks, or tv themes... Tangerine will now let me create playlists based on “feel,” and that’s gonna change my whole working experience.

The app looks very cool too - it looks a lot like iTunes, with album art and a cute little spinning Tangerine as it processes your songs and analyses their beat. I’ve got 14246 songs in iTunes and it only took an hour or two to analyse them all.

Grab the beta yourself at http://www.potionfactory.com/blog/