XML registries are a brilliant idea. It basically goes like this:
You export an XML file from your audio player that contains all of the metadata from every song in your music collection. It's really just a light-weight database that contains a list of all your artists, all your albums and all your songs. It knows how many Pavement albums you have, and that you've listened to Bows and Arrows more than any other album. It knows how often you've played, and how you've rated, each track in your library.
You then setup an account with an online registry, and upload your XML file. Every 4 Gigs of music you have accounts for about 1 Meg of XML data, so be warned, it can take a while. For instance, my collection which weighs in at about 53 Gigs, generates a 14 Meg XML file. Once the registry receives your XML and parses it, there are a lot of really interesting things you can do with that data.
Musicmobs has taken the power of your own listening history and applied it to the concept of social networking. Basically, it knows your taste in music and is able to accurately suggest other people that have similar taste. It's a great way to discover new music, and other people who are into the same shit you are.
As an extension to it's online interface, Musicmobs has created Mobster, an OSX app that runs alongside iTunes that automates the whole process. It can handle the upload procedure for you and it will suggest other artists to check out. What makes it really clever though, is the Hipster to Mainstream slider.
Because it knows the specific tastes of thousands of people, their system is able to see which artists are the most popular and which are enjoyed only by folks out on the end of the bell curve. What the slider does is allow you to filter in real time, the suggestions it's making, based on whether the system views a given artists as being Mainstream or Hipster. Brilliant.
A few other concepts that are equally compelling:
Audioscrobbler, an audio player plug-in that does pretty much the same thing, the only difference is that your remote database is updated every time you play a song, as opposed to manually uploading periodically.
The Internet iTunes Registry, will tell you "More than you ever wanted to know about what you listen to." It's claim to fame is that it can generate a million different charts, visually showing you your listening habits.
Last.fm, isn't based on XML, but it is in similar territory. Their master plan is to create internet radio that is tailor made for each listener. They figure: if their system knows what you like, and it can find other people that share your taste, and it knows of tracks that they love, but you don't have yet, then it's pretty easy for it to give each listener a constant stream of music they haven't heard but will probably like. Genius.