I've highlighted Khan Academy a few times over the past few years, but thought I'd do so again since the site has been recently reorganized and looks easier to use.
There is also a free curriculum, or program, that our children are using exclusively as their math program. My husband, who has a minor in math and is a math whiz, was impressed with the program, as it allows a student to move at his/her own pace. Once you've completed 10 correct answers (a "streak"), you can move onto the next concept. A knowledge map is there, for you to check on your student's progress.
Salman Khan (Sal) founded the Khan Academy with the hope of using technology to foster new learning models. He is currently an investment professional in Palo Alto, CA and has held positions in venture capital, product management, and engineering. Sal received his MBA from Harvard Business School. He also holds a Masters in electrical engineering and computer science, a BS in electrical engineering and computer science, and a BS in mathematics from the Massachusetts Institute of Technology.
Many testimonials are posted on the Khan Academy site, including these:
"I am a high school Math/Physics Teacher working in Toronto Ontario. I have been referring students to your videos for the past year and have referenced many of them on my website. Those that have used it find it very useful, and have commented that you explain content better than I do..."
"...Your videos have become very popular in my school, and my grades have sky-rocketed ever since I became accustomed to your methods of teaching..."
"I've never been very successful with algebra, I know this is pretty basic, but I have almost no comprehension of mathematics. I was honestly going to be kicked out of school for being non-productive, I was ready to be thrown out, but now I get this. Thank you so much..."