K'Nex Computer & Calculators! Answered
Check it out: some students Olin College have been building calculating devices entirely out of K'Nex! These are totally fantastic old-school devices, ball-powered. My inner geek is having fits of joy.
Check out some pictures of their stuff here.
Unfortunately they don't have complete instructions up! They should definitely post the whole thing on Instructables- I sent Matt a message earlier today.
If you want to see his project on the site, drop Matt an email and tell him he should post the directions on Instructables! His contact info is here.