Eagle CAD is quite famous (linux, mac, windows versions available) as program to create printed circuits boards, for many reasons:

• there is a 'demo' version fully working with only some limit (size 100x80mm and only two layers);
• is complete of autorouter and schematic editors;
• include a huge library of components and allows an easy self-creation library whenever a component is not included in the main library;
• has a price quite affordable if the demo size is not sufficient for the job;
• it is really easy to learn;
• include many scripts and cams to automate exports (yes, even gcode, directly) for milling or standard formats for the board makers.

The realization of prototypes using eagle and a milling machine can be done easily and quickly. Please do not understimate the quality of a milled pcb. Using the right machine and the right end mill the result can be a pleasant surprise.