Here is a list of several websites that I have found useful in the classroom.
Math Facts:
http://www.aschool.us/math
http://www.oswego.org/ocsd-web/games/Mathmagician/mathsmulti.html

Spelling:
http://spellingcity.com

Social Studies:
U.S. State Identification Activity
Flash based activity where students match the name of one of the United States with it's location on a map.
U.S. State Location Activity
Drag and drop a state to it's correct location within the U.S. This website also contains many other activities.