Eagle CAD is a PCB design tool used by many hackers around the world. It can be used on multiple operating systems, free for non-profit use (although a little bit limited) and has a nice learning curve. It has some drawbacks but if you are designing PCB's for a relatively simple project it is a nice choice. It is quite a detailed program and I'll divide it into 3 parts for focusing on different aspects of it. Part I is about using the schematic editor, part II is about board layout editor and part III is about library editor.