Really depends on whether you're just looking for visualisation and vague layouts, or accurate engineering drawings with the possibility of manufacturing parts directly from them or having someone else make them for you, possibly with a CNC machine?

Google Sketchup or whatever works fine for the first, for the second you'll be looking at learning a full-on CAD package (which I'd recommend anyway, though it's a lot more effort).

Personally, I use Autocad for 2D DXF stuff and engineering drawings, and Autodesk Inventor for 3D - everyone else here will have their own preferences though. Solidworks and Rhino3D are also commonly used.