Subject Directories:

BUBL Link / 5:15  -- a catalog of selected Internet sources covering academic sources and cataloged by Dewey Decimal classifications.  If you are used to library research, this subject directory will be easy to navigate. 5:15 refers to the results received-at least five sources will be posted, with a maximum of 15 resources for each subject.  This site was created and is maintained by librarians in the United Kingdom.  An excellent tool for middle school and beyond. 
 

Librarian's Index to the Internet    -- reliable, librarian-selected resources to the Internet.  Each site posted is reviewed at least two times before added to the database.  Librarian's Index to the Internet includes a What's new section, and a thematic section.  This site was created by a librarian and is maintained by librarians in California and Washington states.

KidsClick!  -- maintained by the librarians who bring you the Librarian's Index to the Internet, KidsClick! is a subject directory for younger students.  It has an advanced search feature like it adult counterpart, but the site is child-friendly. This is a good starting point for elementary and early middle school students.