Very cool.

So, is there a philosophy behind these plans? economical but sturdy? somehow environmentally friendly?

I'm wondering why they would bother.

I've got an old, poured foundation with 3' walls that is probably 25' x 25'... just sitting in my woods. I'd love to build something like a cottage on it.

Kelton, read the first link (Introduction) on that page and it will give you some background. The reason they do it is because they get paid to do it as part of the national extension program. The extension offices are a huge resource (sometimes more valuable/knowledgeable than at other times) for local gardeners, farmers, nurseries, plant hobbyists, homeowners, etc.