It is a sad, sad truth that in my experience open source, Linux based 3d CAD software is not nearly good enough for prime time. As a result, i have spent quite a bit of time using AutoDesk Inventor and SolidWorks 3d CAD software under a variety of Microsoft operating systems.
Interestingly enough, both AutoDesk and SolidWorks have just set out what they call 'engineering stimulus packages' so folks who have been negatively impacted by the economic downturn can download, train, and use their software for free. Pretty cool, actually, as these are the two best 3d CAD packages i have ever used. You may have to demonstrate that you are actually unemployed or a student, unfortunately, but it is still a pretty nice option to have. And free training is even nicer.
The AutoDesk plan can be found here, and the SolidWorks plan here. Enjoy!