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.