A couple of students from Olin College did this back in 2006/2007 with a 4-bit adder, no electricity, no motors: <a href="http://knexcomputer.blogspot.com/" rel="nofollow">http://knexcomputer.blogspot.com/</a><p>It's up in the publicly-accessible library in Needham, MA if you live nearby and want to check it out!