Kicad is the software we use for designing PCBs.

For linux, just run

sudo add-apt-repository --yes ppa:js-reynaud/kicad-4
sudo aptitude update
sudo aptitude install kicad kicad-library

For Windows,you can download it here.

Below is a video with a short introduction to KiCAD.

On linux, when you try to open up stuff, it may say you're missing a ton of libraries. To fix this, launch EESchema, then go to “Preferences→Component Libraries”. Add `/usr/share/kicad/library` to the user defined search path.