KidsDomain has put together a massive list of freeware for kids, sorted by subjects such as Art, Science, Math, Creativity, etc.
There is a lot of stuff here, and it’s all free. Try building a train diorama, or printing out one of dozens of free coloring books, or check out the free card games (Caveman Cards, anyone?). This would be a great site to visit with the kids for a few rainy/snowy day activities.
I am so all over this. If Kids Domain can keep kids occupied in a healthy, educational, and creative way, they certainly have my support. I will be quite pleased to give them a try, and will probably blog the results, knowing my general geekitude.