Here are some websites that I find useful...
Chapter Quizzes for our Grade 8 Math Textbooks
National Museum of Virtual Manipulatives
Maw and Paw Kettle Math
Archie Bunker Math
Abbott and Costello Math
Useful Math Sites
Johnnie's MathPage
Fun Brain
Cool Math 4 Kids
Math Playground
Multiplication.Com
Figure This
Math Cats
Kids Numbers
Illuminations
Learning Games For Kids
CrickWeb