White House for Kids http://www.whitehouse.gov/kids/whlife/index.html
National Geographic for Kids http://www.nationalgeographic.com/kids/
Starfall.com is full of literacy activities. http://www.starfall.com/
This site provides plenty of practice with upper and lowercase letters, numbers, and shapes. http://literacycenter.net/play_learn/index.htm
Another site that provides songs to help with grammar rules. Between The Lions