I thought people here might find this interesting. This is an outline of "Design Principles for Open Farm Tools, and Questions to Consider". It's from FarmHack, where a bunch of people (who are designing, prototyping, or making homestead or small-farm tools and equipment) post pictures, diagrams, how-to, and text on the subject.
http://farmhack.org/forums/some-design-principles-open-farm-tools-and-questions-consider
And this is their page indexing such tools:
http://farmhack.org/tools