Yeah, never ever touch sketchup for 3D modeling/printing. Shit is terrible, doesn’t support STLs natively, doesn’t export manifold shapes, etc.
People who mention pro software as CAD software alternatives should be obliged to mention their pricing. SolidWorks starts at $4000USD for a single license, Rhino is $999. Who has that kind of money to spend on their hobby in addition to RepRap vitamins? 
Not everybody is a student. On the other hand, IMO using proprietary software with open hardware such as RepRaps kind of defeats the purpose.
@FreeCAD is an open source parametric, history-based 3D CAD modeler running on Windows and Linux. It also runs on Mac, but that version is buggier and not as up-to-date. The @LulzBot people use it along with other software for their designs. I show here how it can be easy to edit a part from its base sketch. https://plus.google.com/117324667497119008450/posts/Hr56gqzuwzD
FreeCAD is far from being as complete as SolidWorks or the like, but it is quite sufficient to model mechanical parts for 3D printing. Modeling assemblies is a little more of a hassle but doable (and not more complicated than in AutoCAD). Anything more “organic” in nature I would say go with Blender.
Rhino is fully useable for 25 saves, the student version is 180$. (including just a single class at your local tech college) Besides great 2D and 3D features it also has free add-ons like Grasshopper and Firefly that allow you to input data from your arduino, text files, spreadsheets etc. My only complaint is that it is not parametric.
BTW the student version is yours forever and does not leave watermarks or overlain images on your files, omit lines or anything else.
+1 for tinkercad. Sketchup is also possible but needs plugins to correctly export manifold meshes for 3dp. And if you can handle it: open scad or blender.
Thank you all for the tips. Several good pointers.
I am doing modeling mostly with my kids and we started with Tinkercad but the low resolution (facets) made us move over to 123D. I can handle SCAD but my 7-year old son is not really ready for that yet.
@Peter_Parnes
I’m 45 and I don’t think I’ll ever be ready for OpenSCAD.
How about http://shapesmith.net/ ? I haven’t tried the latest version.
OK, #shapesmith .net has lost a lot of tools with the redesign. It only does CSG for now (parametric!), but the author says that the refactored code will scale better.
Blender