One of my favorite book sites is LibraryThing, an online book cataloging and social networking site. You can catalog up to 250 books for free and after that a life time membership is only $25. One of the coolest things about the site is the "Suggester." With the suggester you enter a book you like in the search field and LibraryThing will compile their massive amount of data to show you what other books a person is most likely to own if they own that book. It's a great way to find book suggestions. LibraryThing also has a Local section that shows readings and information about book stores and other book venues in your area. And Porter Square Books uploads our inventory to LibraryThing, so you can see if we have the book you want.
Another cool site I heard about is called BiblioTravel. BiblioTravel is an online database of books organized around the locations the books are set in. It's a great tool for travelers looking to get a sense of a place through literature. The site is still fairly new, but as books are added it will become a great bookish resource.
And if you haven't been to IndieBound yet, I think it's time. They've even created an app for those of you with iPhones.