Free and open source PCB design software

gEDA
gEDA is a suite of electronic design automation (EDA) tools.
http://www.geda.seul.org/


PCB
PCB is an interactive printed circuit board editor for the X11 window system.
http://pcb.sourceforge.net/


FreePCB
FreePCB is a free, open-source printed circuit board layout editor for Microsoft Windows, released under the GNU General Public License.
http://www.freepcb.com/


Kicad
Kicad is an open source (GPL) software for the creation of electronic schematic diagrams and printed circuit board artwork.
http://www.lis.inpg.fr/realise_au_lis/kicad/


Gnome Assisted Electronics
An application for schematic capture, simulation, and PCB.
http://gael.sourceforge.net/