I know there are probably some threads about with these sorts of things but I'm lazy...

I thought it would be great to have some ideas of freely available curricula that are interesting and at least mildly challenging! I would appreciate any additions anyone would like to make.

Maths:
http://www.c3.lanl.gov/mega-math/
This sis great for activities that can be tailored to the level your child is at. I've used it successfully in classes with children from 5-11 years old.

Science:
http://msnucleus.org/curriculum/curriculum.html
Set out in stages, starts with real science quite early. A recent find I haven't yet used much, but looks good.

http://www.jpl.nasa.gov/education/index.cfm?page=86
There are some less challenging stuff here, but some of it is quite good.