Tell them what titles or genres you have enjoyed in the past, and they will give you surprisingly insightful recommendations. Make friends on Goodreads, too! Chances are your friends are discussing their favorite (and least favorite) books on Goodreads.
Library Thing calls itself the world's largest book club. Enter all the books you have read and find more you might like.
This simple search engine interface gives you quick recommendations based on a book you have enjoyed.