If you're going to use it for 3d printing then AUtodesk Fusion 360 is really great as it contains a traditional volumetric CAD and also a modeler in which you can design organic shapes, all that integrated into one nice package.

You can also import stl files and design around them. Of course you can export to stl.

Free for students, enthusiasts, hobbyists, and startups

I've been using it for 3 years now, really a great tool, easy to learn, very intuitive.

http://www.autodesk.com/products/fusion-360/try-buy