VectorKids is a site that offers several interactivities and games to help build student skills in Mathematics (and even Social Studies too). The site offers mostly math activities to help students with basic to intermediate math concepts.
VectorKids skill builders include help for multiplication tables, solving for variables, division rules, addition, subtraction, counting money, geometry and even a United States Capitals Game.
The games are simple and would offer a nice extension to your classroom activities or extra help for specific students who may be struggling with these concepts.
Check out VectorKids at: http://www.vectorkids.com