I used MoodLogic for awhile to create playlists tailored to my moods, and it performed pretty well, but I wasn't really satisfied with the limitations of the free version. MusicIP Mixer is a free Windows app in the same vein. You choose a song from your collection and it will generate a playlist of songs that it deems similar. While you can start generating playlists as soon as it's installed, the initial algorithm, though fast, seemed wanting to me, and the subsequent (and presumably more accurate) algorithm takes a long time to analyze your songs. It does have some nice features, though, like integration with other players so you send generated playlists to Winamp or iTunes, and compatability with portable devices. It also has a built-in music-recommender that will query the web for new music you might like. Though I haven't been using it long enough to really judge the quality of its algorithms, MusicIP Mixer has a nice polished feel and is worth checking out if you're sick of assembling playlists by hand.

[Thanks, Tim!]