Tucows lists ElastoLab, FreeCAD
Have a look at this Tucows listing for ElastoLab, and this one for FreeCAD. Both get 4 cows (on a scale of 1-5). Very cool. I haven't used FreeCAD, but hear good things about it. ElastoLab I can personally recommend - my daughter liked it, and my nephew was enthralled.
