I too used Purple Algebra, long with two other sites that are excellent.

Math.com

http://www.math.com/homeworkhelp/Algebra.html

and

Coolmath.com

http://www.coolmath.com/algebra/01-exponents/index.html

My favourite is Coolmath.com, because of the breakdown, step-by-step, that does lead to a deeper understanding.

I used all three, because it is relatively easy to copy and paste, unto flash cards and than hit the print button. If my child is struggling, I than will print off the lesson, as well as practice problems to work on and for studying purposes.

All three sites are of good design, where it is relatively easy for a parent to understand concepts, that they have long forgotten. Coolmath is the best one using fun and easy language to understand the concepts.