SJ highlights several of the most popular web-based tools for finding new books:
Goodreads.com is quick and easy. Yournextread.com is fun and helps a lot. But I gotta be honest, Amazon's suggestions are probably the most useful to me.
TheFu suggests checking out award-winning lists and one rather quirky way to pick a good Sci-Fi book:
For scifi, see Hugo winning books. Life is too short to read bad books. Sometimes that leads to an author with an entire series of books to enjoy. I really enjoy some of the scifi from the 40s and 50s. Wells stuff is always timeless too (and free). I'm less happy with Nebula winners–-different type of writers and not my personal taste.