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.
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…"