As advised by Ralph Reed i got lernsys academy for my son. Its just really awesome. Lernsys homeschooling is site with step by step explanations and I have been pretty impressed with it. Take a look at http://www.lernsys.com/en/grade-8-math-l...-quiz-exam. They have free preview too.
You might also take a look at Aleks.com, Khan Academy, or CTC math. We've used them all at different times.