I'm probably the last person in the Omniverse to find this super cool site, but, just in case you've missed it too, here's my trumpet to the world.
Library Thing lets you create an on-line catalog of your library. Each personal library is linked and cross-indexed to other personal libraries, which allows for a nice exchange of book ideas and reviews. Though adding every book in your library--if you have oh say about 5,000 books like me--is rather time consuming, Library Thing makes the process pretty easy.
Many authors have libraries on Library Thing, and though it's cool to see what absolute strangers are reading, all the more interesting to see what your favorite authors have on their shelves.
My user name is Ysabeau, if you want to look me up, and I'll try to link here once I can figure out how to do that! In the meantime, start cataloging!