I got introduced to LibraryThing since June 2007 and have been using it to catalog my knitting books. I appreciate the simplicity in use and ability to add tags to help me keep track of items in my collection. Currently my collection of my cataloged books is only 131.
I've actually gotten serious in how I tag my knitting books. I have a running list of the thought progress so I can keep the tags consistent. You can tell I work in a library, eh?
The social networking aspect of LibraryThing is quite amazing. I'm able to see who has the same books in their library that I have in mine. I can also view my tag "clouds" that show how many times I've used each tag by size, a great aspect for a visual person like me. There are a bunch more options that I've not fully explored.
I use an RSS to track entries in from LibraryThing's blog. How's that for referencing Thing #8?
There are other Web2.0 places to track/catalog your books. GoogleReads is one and Shelfari is another. I've not used either of these. Have you? Why do you like them?