OpenDesk.cc also worth a mention! Same concept but for furniture. One of the interesting engineering challenges we faced there was the 0.1 - 0.2mm variation in thickness between different sheets of ply. Those small differences can mean loose fitting joints and a very wobbly output vs a set of parts that simply would not slot together. We ended up having to parametrically design every slot so makers could measure their current sheet and select the right thickness.