Merriam-Webster Online: This site features both a dictionary and a thesaurus, as well as word games and a "word of the day." There's also a link to Merriam-Webster's "Word Central," which is specifically designed with children in mind.
National Geographic Society: As one would expect, this site is chock-full of maps and nature photography. In addition, there are interactive learning projects (including a 3-D solar system that you can navigate through your Web browser), news features, and educational games for kids.
B.J. Pinchbeck's Homework Helper: 13-year-old B.J. Pinchbeck has spent many, many hours combing the Internet for sites that can be of help to students, and he's compiled them on his own Web site. There are more than 600 sites listed in more than a dozen categories.
KidInfo: There's information here for students of all ages, but there's a wealth of links specific to younger children. There are also resources for teachers and parents.
Time magazine: Keeping up with current events is easy with this news-filled site. There are full-text articles from the magazine, a searchable archive that goes back 5 years, feature articles that are unique to the Web site, and a link to the Time for Kids magazine.
Bartlett's Familiar Quotations: This site contains the full text of John Bartlett's tenth edition of Familiar Quotations. You can view quotations chronologically by author or search via keyword.
Chem4Kids: This site uses humor and a fascination with green hair to teach about matter, atomic structure, formulas, and other concepts related to chemistry. You can take a quiz to see how much you've learned when you're done with the lessons.
The Complete Works of William Shakespeare: The complete text of each play and poem is here, and you can easily search for words in a particular work or among all the works. There are study guides for the major plays, a timeline of Shakepeare's life and works, and links to other Web sites about The Bard.
The Cornell Theory Center Math and Science Gateway: Aimed specifically at high school students and educators, this "gateway" highlights Web resources in a number of categories, including astronomy, physics, mathematics, agriculture, and more. There are also links to Cornell's other educational gateways.