If you don’t want to create your own playlists, the free MusicIP Mixer application analyzes your library and builds playlists for you by matching songs that have similar acoustic characteristics. Import your music (click Library > Add Songs or select Sync with iTunes), and then choose Library > Start Analysis. When the program finishes, select the song you’ll base your playlist on, and then click Mix.
To tweak the way the program builds its playlists, select File, Preferences and adjust the Mix Settings. The mixes the program created for me didn’t always make sense on screen, yet when I listened to them, they flowed nicely. If you don’t like one of its choices, just right-click it and select Replace.