BookSeer is a simple web-based application that answers the question "What should I read next?" based on what you just finished, and presumably enjoyed, reading.
If you want to do some complex digging into your literary tastes with a variety of metrics you'll want a site like BookArmy. On the other hand with BookSeer you can plug in a book you enjoyed reading and it will return the top recommendations from book sources like Amazon.com and LibraryThing to suggest a list of books you might enjoy. Each book in the recommendation list is linked back to information from the source so you can dig down and read more about the books BookSeer is recommending. Have a favourite tool or technique for finding new books? Let's hear about it in the comments below.