LibraryThing

Calling all geeks! LibraryThing lets you catalog your books online and find people with similar libraries, or get suggestions from people who share the same taste in literature. Tagging makes the world wide web go round and it definitely makes it easy to search for books based on author, content, title, etc.
Here are some stats taken from their super-nifty Zeitgeist page:
Books cataloged: 2,288,631
Total users: 32,317 (since August 29, 2005)
Unique works: 710,505
Total tags: 3,326,124
Total reviews: 27,269
Total ratings: 285,853
User-contributed covers: 60,836
If you don't want others to see your collection you can keep your account private and no one will be able to see your books. I guess if you want to keep some books private but still share the rest of your collection you could set up two accounts… but wouldnt it be cool if you could do that from the same account?