Bigchalk.com - All subjects and grades.
Strength: Current events, and social, political and consitutional issues.
Encyclopedia Britannica for kids (http://kids.britannica.com/) - All subjects
Strength: Kids. may need registration
SparkNotes.com - All subjects and grades
Strength: English department. Summaries, analyses, quotes, discussion...the works!
The Math Forum@Drexel: Ask Mr. Math (www.mathforum.org/dr.math) - All grades & college
Strength: Help with math by college and graduate students, professors, and scientist. No free answers here!
The History Channel (www.historychannel.com) - All grades
Strength: Obviously, history. But it is also interactive with sound and movie files.