Cadsoft EAGLE Schematic and PCB editor
Cadsoft is a German company that is a veritable mecca of software distribution enlightenment. In addition to the reasonably-priced professional PCB design packages ($1200), they have freeware, lite, non-profit, and other intermediate licenses. Their software runs under windows, linux, and MacOSX. It's slightly quirky, with a steep (but not too high) learning curve on the front end, but from most reports it is not any more so than other professional CAD packages. They have online support forums that are active from both the company and other users, the package is under current development and gets better with each release. A number of PCB fabricators will accept their CAD files directly. It's good stuff.
Use it. Propagate it. Buy it when you "go pro."
There are several EAGLE related instructables now:Schematic EntryCreate PCB from SchematicMaking PCBs for HobbyistsCreating Library Parts
In theory, this forum is a place for all the little questions that don't rate an instructable on their own, or a gathering place for info destined to future instructables, and stuff like that.