An Old Fashion Education is (according to their site) a directory of free homeschool curriculum. It is also one of my personal favorite sites. I can almost always find a resource through them. Their list of subjects is comprehensive and easy to use.
Not only are individual subjects plus complete curricula resources available, but they also have 40 week lesson guides for each grade level. You can teach your children practically for free! This is one homeschool resource no family should be without.