In the last two weeks, I have used Khan Academy with two different classes. I was amazed how after teaching the different concepts for two full days (manipulatives and all) the deer in the headlights look changed to smiley faces saying, "I get it!" when the students watched the videos! At first, I hesitated using the videos because of the time frame, but another colleague pointed out that I probably talk longer then the video :)
I would love to hear about others that are using the khan academy sight to help with math concepts.