Celestia is an open source program for exploring space. It isn't exactly a game, since there's no particular objective; you just zoom around and look at stuff and maybe learn something about astronomy in the process. You can travel to other galaxies, but it's based on real astronomical data so our own solar system is far more detailed than anything else out there. I'd been playing with it for a while, but I just realized how much better it looks with some of the alternate textures available from the Celestia Motherlode.
I put my math worksheet generator online. I wrote this program to generate printable math worksheets for my son. For now, the options are pretty limited: it either generates addition, subtraction, or multiplication problems. Each page contains sixteen problems. The addition and subtraction problems use numbers from 1 to 999. The multiplication problems are single digit.
If you want to make work for your kids, check it out here: http://cheznick.net/main/maths