I just found and downloaded SolveSpace and quite like it! I had a specific design problem with a 4-bar linkage, and it was easy to figure out. I wanted a fully dimensionally constrained mechanism (the link lengths and inputs) with a few other reference dimensions (the outputs). The main frustration I has with this was the reference dimensions first had to be created as constraining dimensions, which elicited an alarming red screen. The red screen disappeared as soon as I changed the over-constraining dimension to reference, but I would suggest that over-constraining dimensions be automatically created as reference dimensions, or at least alert the user that the dimension overconstrains the system and give them the option of assigning a reference dimension right away (since new dimensions can be hard to find in complex drawings). This does not change the essence of the workflow for the software, but would provide a little better experience for the user (me in this case ;-)

Thanks for a great tool!