To practice math facts:
Cool Math 4 Kids – Aimed at younger children, this site provides math fact practice and lots of games involving math concepts.
Cool Math – partner site to Cool Math 4 Kids, but aimed at ages 13-100! lots of games involving math concepts.
Multiplication.com – great games for practicing math facts. We especially like the team games, set to “Private”, such as Grand Prix and Diaper Derby!
Math Fact Cafe – a site with simple, clear printable fact practice sheets