So, I've been playing with this program called Soundflavor dj for a couple of days now. It's a (free) playlist generator that works with iTunes. Picked it up after purchasing the now free (grumble) muffin music finder from Magix and playing with the built-in playlist generator in winamp (I crashed it the second time I used it). Both excellent generators but neither can access the iTunes store stuff (For all of you fans of The Filter, I'm still waiting for them to grant me access to the site to download the program; the day I registered there was a glitch in their email system; received an email from them about the problem a couple weeks ago but nothing since).
I have my own smart playlists set up but sometimes they're not exactly what I'm looking for. Soundflavor does a fairly good job of finding similar tracks in my library. It also recommends songs for download (some for purchase, some free) from their website (with free registration). You can also upload playlists to their website to share with others or post to your website/blog.
I like Soundflavor… mostly. It only id'd about 3100 of the 3600 songs in my library. I can make it hang if I switch songs more than twice and although it works from within iTunes it doesn't allow use of iTunes' crossfade featureL. Still, it's better than hunting.