Make hobbyist PCBs with professional CAD tools by modifying "Design Rules"

Step 2 Intro, part 2 - Cadsoft EAGLE

Cadsoft EAGLE: http://www.cadsoftusa.com/

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."

This document is not a tutorial on how to use EAGLE, although it'll probably be somewhat useful in that role. It's more about how to configure and customize an Eagle installation to better suit the hobbyist.
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See also:
Schematic Entry
Create PCB from schematic
Creating Library parts
Design rule modification
Send CAD Files to manufacturers