I've spent the morning listening to music from my iTunes, but selected via The Filter. It's a great tool (free download here), which mixes the taste of thousands (like last.fm) but with a huge difference: it does it with my own music.
At the heart of The Filter is a music recommendation engine that is continually learning the listening tastes and choices of tens of thousands of music lovers. This huge collection of information is combined with your own likes and dislikes to create a personal understanding of what songs from your collection you will like to listen to and when.
It can be also used to create fresh playlists for your iPod, and to get a lot information about the music you are listening. It's great, for free, and with versions for Windows, Mac and Nokia phones.