It depends on what type of music you're talking about. If you mean store bought cds, then I have no idea. I have a list of my music at http://db.etree.org but that website is used to catalouge concerts (like Grateful Dead, Phish, String Cheese Incident, etc) so people can trade shows with each other.
I'm basically just looking for something where I can list all the music I have on my computer online just to make it easier for me when people ask what I have.
Try www.lala.com Really a CD trading site, but you can sign up for free and list your CD's and you're never required to trade. People who know your user name can see your collection if you allow it.
Yeah, I was gonna say try IGN. Though I'm not positive if they have it allows you to catalog music cd's but I know it does videogames and DVD's for sure.
Do these sites generally just list retail items? Or is there any way to account for independent or promo releases, like a form that you can fill out to create custom list entries? I have a lot of promo singles and stuff in my collection, so I would want a way to account for those. Meanwhile I've got my own list on a tripod site where I've just used some manual HTML coding... but something a bit more upscale would be nice!
I use http://musicmademe.com/ . It has lots of CDs in their database, but if they don't, you can always add it manually and/or request the info be uploaded.
You can always use www.gametz.com Any Music CD's not in their database can be listed manually and are then added to the database so you'd be able to list every cd you have.