SketchUp is a premier 3D design software that truly makes 3D modeling for everyone, with a simple to learn yet robust toolset that empowers you to create whatever you can imagine.
Some people may not have noticed the free drawing program which comes with OSX. It's located in Developer/Applications/Extras and it's called Sketch. It's only installed if you do the developer install. It's a simple drawing program, with lines and circles and type, similar to Microsoft Draw. It's nice for simple diagrams when Illustrator is just too much work. The only catch is you have to save your work as a Sketch file (.sketch) before you can Save As a TIFF or PDF file.Editor's note: You'll also find Bomb.app (a program to show that a dying program won't affect the system) and WorldText.app (a TextEdit replacement with some nicer features) in the Extras folder - it's worth a look if you've installed the Dev Tools.