I am surprised that I never listed AAAmath.com before. I had been using it as long as I have been homeschooling to help the kids figure out concepts in their math books or online program that was not clear. This site does a great job of explaining difficult concepts clearly.
AAA Math features a comprehensive set of interactive arithmetic lessons. Unlimited practice is available on each topic which allows thorough mastery of the concepts. A wide range of lessons (Kindergarten through Eighth grade level) enables learning or review to occur at each individual's current level.