Texas Instrument Education (calculator):

http://education.ti.com

General drill/practice/brain stretchers:

http://funbrain.com/kidscenter.html
http://aplusmath.com
http://gamequarium.com
http://puzzlemaker.com
 

Social studies:

http://nationalgeographic.com
http://cantonpl.org/kids/country.html
 

General spot good for teachers and parents:

http://edtech.kennesaw.edu/web/teaching.htm

 

For current events appropriate for kids:

http://www.kidsnewsroom.org
http://www.timeforkids.com
http://www.sikids.com