I enjoyed this post on Lifehacker a while back.  It features some good sites to keep in mind for reference work.

Little appreciated outside the world of academia, there are literally thousands of .edu sites bursting with incredibly useful and interesting information and resources. Most of these sites won’t pop up to the surface of the average search engine quest, and so they wait, neglected and underused…until now. Keep reading for a quick tour through the mysterious underground world of .edu.