SweetSearch

Sweet Search is an educational search engine where all the website that it suggests has been evaluated by a research department for educational content. A simple way to help reduce extraneous search results, especially helpful with students doing their first online research assignments.

They also have:

SweetSearch for School Librarians for content that helps students use the Web effectively. 

SweetSites for teachers and students, organized by subject and academic level.

SweetSearch Biographies for profiles of 1,000+ significant people.

SweetSearch Social Studies for our best social studies content.

And a great article on Ten Steps to Better Web Research.