Tangerine!

Tangerine! is an iTunes add-on that lets you create custom playlists according to the songs' BPM (beats per minute).

If you're going for a run, Tangerine! can help you create a playlist with a high BPM. On the other hand, if you want to relax, you can use it to create a slower playlist with fewer beats per minute.

You can select how long you want the playlist to be, as well as the beats per minute in the songs, or their intensity. You can also choose the pattern you want your songs to follow: ascending, pyramidal, random, etc.

If you don't like a song or think it doesn't fit the desired BPM, you can easily change it and select a different one. All you have to do is drag and drop.