I would consider the Illustrator pen tool to be one of the most impressive, and difficult to master, tools in the entire Adobe suite. With it, designers can create countless lines, shapes and illustrations. I normally wouldn't highlight an individual tool here, but it's important that designers get comfortable with the pen tool, and hopefully go on to master it through practice. Once you get the hang of it, the power of the pen is only limited by your creativity.

How do you use the pen tool? Examples are welcome.