A colleague recommended the Cool Math website as something she uses as a supplement for her students. There are some games for the younger kids, and lessons in pre-algebra through algebra for secondary students. I’ve glanced through a few of the pre-algebra lessons, and like them. They’re short, fairly clearly written, and have good diagrams.
The site is also friendly to homeschoolers and their parents, with a decent teacher’s area that outlines the author’s perspective on teaching math.
Their Survivor Algebra is an interesting approach to encouraging peer teaching by breaking the class into “tribes” and giving bonus points on tests if all members of the tribe do well. I’m not sure I like the extrinsic motivation of the prizes and test score bonuses but I think there might be some good aspects of this type of classroom organization in very large classes.
It’s a very interesting site that’s worth investigating.