My two favorite open source 3D design tools:
Uses an object description language to provide all that 3D goodness. You don't so much draw objects as program them. An object can be viewed in 3D whilst it is being developed and can be easily exported to an .stl file for manufacturing. The Openscad language offers excellent parametrization features - so you only have to invent the wheel once. Available on Windows, Linux and Mac. Extensive documentation.
I can hardly describe how far this tool has come in the last 6 months or so - simply brilliant! - There are loads of tutorials and documentation. FreeCAD also supports parametrization (IMHO it's complexer than in OpenSCAD though). There are a plethora of Modules and Modi and the learning curve is steeper than Openscad. FreeCAD offers traditional draughting features but has a lot of tools designed to simplify the design process - such as the ability to set design constraints on objects (eg. "these two lines must be parallel" or "the center of this circle is always on this plane at the intersection of these axes"). FreeCAD does have some stability issues though. This is a shame as the program is otherwise amazing.