That'd be cool. I think it's called Google Sketch-Up. About as close as it gets really, and it's not open-source, but it's free. I personally pirate software that's useful to me, but since I'm in school I get all the school's versions free anyways, kinda mute. I see your point and need. It's a market that software hasn't filled yet. I suspect it will be, but the software won't be nearly as powerful as the current market leaders (autocad and solidworks come to mind).
Problem is, those who need are willing to shell out. Those who want to tinker don't need the enormous complexity those programs offer, they want something simpler. Sketch-Up does 90% of what most tinkerer's need.
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