This is currently what I'm struggling to find. The main thing I've established is FreeCAD just isn't ready yet - very buggy and I can not get it to work, but parametric modelling is an interesting concept.
What else are people using for dimensioning parts which need to fit together? (i.e. part design, rather then modelling I guess?)
I've been using freecad, personally. I just did a series of adapters that allow me to attach RC servos to LEGO bricks for some inverse kinematics robots. It worked reasonably well. I've had it crash and do unexpected things, but what I've found is that if I work in the part design toolbar, build sketches that are fully constrained, and then use extrude/pocket operations to build my final parts, it seems pretty robust. Then I can switch to mesh and turn those into exportable meshes individually, and get parts that interact the way I want to. Half the stuff I'm doing I 3d print and the other half I mill on my cnc mill, and when I'm processing stl's I find again that having started from fully constrained sketches means the stl's are more robust and less likely to crash the cam programs I use.