Okay, this site is super cool: Library Thing. You enter in your book collection, then you can see who out there has the same books as you. I’ve only entered 7 or 8 books, but I’ve already found several kooks who’ve read both The Pelopponesian Wars and The Faerie Queene.
Even better, since I spend a lot more time reading blogs than I do books, would be the equivalent concept but for blogs: a place where you could enter all the blogs you read, then find people who are reading similar blogs. I suppose bloglines comes close to this ideal, but it’s not quite there.
The only other problem with Library Thing is that you very quickly find out just how many people are more well-read than you are.