[EDIT Oct 2016: changed link to Reading Library to new website]
I’m a frequent user of Project Gutenberg, which distributes free eBook versions of books in the public domain.
Be A Learning Hero, a site I administer, has great lists of suggested books for kids to read at each grade level, and guides to aid parents in discussions with kids about those books. In working on getting that launched this summer, I wanted to know if any of our books were available at Project Gutenberg, so we could just link to them (none were, but we added WorldCat links to make it easy to find them at a local library).
Anyway, while working on that, I realized that there is no convenient online list of free books for kids, organized by grade level. There are lists of public domain books for kids, and there are lists of books for kids by grade level, but nobody seems to have combined it all in one place. So I thought: SEO gold!! And then I put this together.