Thanks to the efforts of Thingiverse superusers like Michael “Skimbal” Curry, Trimble SketchUp (formerly Google SketchUp) has been the tool of choice for some of the most popular items from desktop 3D Printing. (See the Gothic Cathedral Playset below.)
With its intuitive “direct modeling” style of control (you will see ideas like these explored in 123D Design) and powerful free version, new 3D designers world over often get started in the craft using this tool. This app has a training system built right into the software itself, to quickly introduce new users.
Be mindful that the output from SketchUp was initially designed for rendering only — your model might need repair before it is ready for printing. There are a number of tutorials online to help you make sure your model is manifold when you export it.