Guzzlefish.com is a cool idea. From their about page:

Guzzlefish is a free personal cataloging system that allows you to manage and organize your movie, music, and game collections on the web. An easy to use interface not only keeps track of what you own but also keeps you up to date on new titles not even out yet.

They also have ways to review the media in your collection and to sell items that you no longer want. It appears to be what I’ve been meaning to do with my online CD collection but never have.