Srimech (JimM) is building a mechanical Turing machine. At the moment, this is a few full days work away from working, and will probably then need several days of fine tuning to make it run for a long period.
The plan is to make a machine capable of universal computation. It will be hilariously slow however.
4th Dec 2010: First test run. Direction line detached, electric. Completed cycle in 2'14". Faults: Moved forward only one grid position, ball bearing continued past finish row.