AVRs are the heart of a great many hobby and commercial projects. A company I'm associated with has their entire product line based on AVR chips. They're cheap, good, easy to learn, have an FLOSS tool chain, and good debuggers.
If I were working on one, I would base it on the RepRap design (
http://reprap.org/). They're working on build-at-home 3D printers, which are basically a claw machine with the claw replaced with a plastic extruders. They've got good plans, parts, suppliers, and a great community for supporting each other. It's hours of entertainment watching them (collectively) build their own little von Neumann machines.