Joining the family of iTunes plug-ins, The Filter is a free program that works like your own personal DJ by automatically creating playlists from your library and syncing them to your iPod with just a few clicks.
Selecting a few songs directs the program to analyze your picks, scan your library and generate a mood-matching mix. The time-saver will even compile data on your musical tastes to recommend new music it thinks you might like.
I've only been using Filter for a short while but find it perfect for the gym as well as bedtime mixes and has also uncovered old music that I forgot about. Available for Mac, Windows and select Nokia mp3-enabled phones, it is available for download here.