There are a number of programs that generate playlists automatically according to your "mood".
The annoying part is that they all generate and use proprietary data. So basically you have to pay them for data that you help to generate--which is stupid.
MusicBrainz has the data and the capability of recognizing music via TRMs and is open source (or public domain). Thus it has the potential to be an alternative.
There is AudioScrobbler. They do co-operate with MusicBrainz.
I have yet not found out in what they co-operate exactly. Well RichardJones is on the MailingList and aparently Audioscrobbler uses parts of the MusicBrainz data. --DonRedman AudioScrobbler now produces FOAF data and is planning on exporting a lot more RDF for items like "similarity between artists". --DanielOconnor
PetervanHardenberg has started to work on the MixOMatic. This could become an implementation of this idea.
Commercial offerings in this area include MoodLogic and MusicMagic.