Recommended sites:

Fact Monster - Online almanac

USA.gov - Official U.S. Government Web Portal

Who2 Biographies - Famous people, characters & creatures

Dictionary Sites:

One Look Dictionary - Provides results from multiple online dictionaries

Etymology Dictionary - Word origins

Synonyms & Antonyms

Other good sites:

Ask.com

KidsClick!

Internet Public Library for Teens

Awesome Library for Teens

ERIC - Education Resources Information Center