Here's a really cool game I think a lot of people here would like:
http://sineridergame.com/.
Basically, in each level, you're given a set of targets you need to pass through, and your goal is to make an equation for a graph such that when you are subjected to gravity, your dude will pass through the targets. You can't control the dude except by writing the equation for the graph, so he'll just fall down on top of the graph and go (I don't know if I'm explaining this very well...). You can also make the graph change over time (the equation is in the format y=f(x,t) for some function f).
My main complaint about it is that the collision detection seems kind of wonky (I'm not sure how they could fix that though, given how flexible the graphing capabilities are), and sometimes that messes up what you know should be a legitimate solution. But it's a pretty cool game, and I also think it has a lot of educational potential to teach kids about graphing.