SolveSpace - parametric CAD program. Single executable, well done UI, small code base, nice constraint solver, originally by a single person. It's the only OSS CAD I've found useful (QCad was second, then FreeCAD). There are also a few active developers now with major improvements on the way.
Be sure to get the latest from whitequarks repository. He's been doing (or integrating) most of the recent development including the GTK3 and native OS X ports.
