The Arts:
National Gallery of Art: http://www.nga.gov/
The Metropolitan Museum of Art: http://www.metmuseum.org/
The Louvre:
http://www.louvre.fr/llv/commun/home_flash.jsp?bmLocale=en
Classical Net: http://www.classical.net/music/frames/
The Internet Movie Database: http://www.imdb.com/
American Film Institute: http://www.afi.com

English:
Common Errors in English: http://www.wsu.edu/~brians/errors/
A+ Research Writing: http://www.ipl.org/div/aplus/

Math:
The Math Forum Student Center: http://mathforum.org/students/
The Prime Pages: http://primes.utm.edu/
AP Calculus Problem of the Week: http://www.mecca.org/~halfacre/MATH/calc.htm

Science:
Periodic Table of Elements: http://www.webelements.com/
StarDate: http://stardate.org/

Social Studies:
The History Channel: http://www.history.com/
Primary Documents: http://www.freerepublic.com/focus/f-news/1294965/posts