Mac OS X only: Freeware app Tangerine analyzes the beats per minute (BPM) and beat intensity of the songs in your iTunes library, then helps you create playlists based on these attributes.
That means that it can be relatively easy to create an upbeat workout/party playlist, or, alternately, a relaxing, low energy list. Tangerine is quick (it analyzes up to 3 songs per second, according to their site), and it's very simple to use. I'm always looking for new ways to put together a good playlist, and I can say already that I like it. If you're looking for a playlist generator for Windows, check out previously mentioned The Filter or MusicIP Mixer.
