I've used my printer to make some parts for my robot project (full-sized R2-D2). There's been several times I have needed to make shims, various sized washers/spacers and mounting devices for Arduino UNO's inside my droid (electronics and bare aluminum produce expensive blue smoke!)
I haven't found the right CAD software I really like and usually Google Sketchup gets me buy. And of course, I've used items from Thingiverse for stuff already designed.
McMaster-Carr is offers many of their parts offerings in CAD files, which can even be printed! Great of prototyping with my droid