Shelfari is a Seattle based company that launched quietly last fall to compete with Library Thing, a commonly seen widget used on reader blogs accross the net. Shelfari allows you to input your books, rate them, and show them off on your blog. You can also enter your amazon associates id and earn money off the books on your shelf should a user click on the widget and then purchase the book.
Amazon recently invested $1 Million in Shelfari. It would be great to see integration between the two such that any Amazon purchase was automatically entered into the Shelf. The downside to these types of databases is that it is only as good as the information the user enters which can be quite a hurdle for readers with big libraries. We need to have a low priced bar scanner to assist in the cataloguing of personal libraries (not to mention how nice that would be for insurance purposes).