The Book Seer is a neat book recommendation engine that I discovered few years ago through Kristen Swanson's Teachers as Technology Trailblazers blog. The Book Seer is very easy to use. To get a book recommendation just type in the title and author of a book that you've recently read and the Book Seer will spit out a list of related titles and authors that you might enjoy. I tested the Book Seer with four different titles. The more obscure titles that I searched for, Snow in the Kingdom, and A Good Life Wasted didn't yield any recommendations. When I searched for The World Is Open and Hatchet plenty of recommendations appeared.
Your Next Read is a neat little site that provides you with a web of book recommendations based on the authors and books you already like. Here's how it works; type in the title of a book you like or author you like and Your Next Read will provide you with a web of books that might also enjoy. Click on any of the books appearing in the web to create another new web.
Compared to the Book Seer and Your Next Read 2Titles takes a slightly different approach to making book recommendations. On 2Titles you answer a series of eight questions about your personality and interests before answering questions about books you've previously read. 2Titles will try to prompt you to create an account, but that is an optional step that you can skip if you can find the "skip" link.