I am really liking Library Thing. At first glance it looked like a pointless bit of busy-work, inventorying all the books I read -- even the ones I didn't like. But then I found the Recommendations features. Library Thing will show me other reader's collections with whom I have a lot of books in common. I can virtually browse their shelves and find new books to read. Library Thing will also keep track of books I liked and recommend books that other readers have listed as good pairings.
Nowadays, instead of being chronically short of things to read, I have a long wishlist and have found several new favorite authors.