I haven't had much time to muck about with it, but I came across a nifty new (well, it's not *that* new, but it's new to me) site called LibraryThing. It's sort of a book collecting / cataloguing site, combined with a social network site like MySpace, as it allows you to find other users of the site with similar tastes in books.
Though there are other cataloguing programs available (and I've mucked with some in the past) there's something sort of cool about being able to maintain your collection online at a consistent web address. Makes it easier when someone says, "Hey, you got any good books?" You can just answer, "Here, check out my collection."
Which you can, in fact, do here.
It's a work in progress -- most of what's up there is just from memory, and even then mostly my more recent acquisitions. Still, it gives a good feel for what the site can do.