KiCad is a popular open source printed circuit board layout program that runs on all three major desktop OS's (Windows, OS X, and Linux). It was originally written by Jean-Pierre Charras at Université Joseph Fourier and is now actively developed by an enthusiastic team of developers. KiCad is quite capable of producing production-quality designs and has been my EDA tool of choice for several years.
Like all PCB layout programs, KiCad has a steep learning curve. So here's a few things that I am hoping might help ease bits of the curve.