MusicIP Mixer is a free Windows app where 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, is fast but seems to be lacking, 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 can 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. MusicIP Mixer has a nice polished feel and is worth checking out if you're sick of assembling playlists by hand.