Found this sweet little tool called OpenDB on Sourceforge.net. It's an insanely flexible cataloging system, allowing you to define whatever types of objects you want. It comes with several pre-defined types (DVD, VHS, CD, Laser disc, VCD), but oddly enough no book type. You can define users and assign the ownership of each item to a user. If you're always lending things out to people, you can keep track of that too.
Probably the best feature of the program though, are the plugins that let you do lookups against sites like Amazon.com, Internet Book List, FreeDB and IMDB. For books, all you need to do is enter the ISBN, do the lookup on Amazon and it fills in all the fields you've set up for the Book type you define. For other stuff, just enter Amazon's ASIN and look it up. Entering a title will get you a list of possibilities. Click the one you want and all the fields are filled in for you. It's awesome. No need to type in anything manually, which is probably the biggest barrier to create and maintain a comprehensive inventory of your stuff.
Runs on PHP and uses MySQL on the back end. The database schema is fairly small and compact for what it stores and tracks.
If you've got a collection of stuff you're looking to keep track of, this is a cool bit of software to check out.