Fritzing is an open source free downloadable PCB design tool for hobbyists that only allows you to design and export gerbers for two-layer boards. It runs on Linux, Windows, and Mac:
I recently created my first “real” printed circuit board. By “real” I mean one that I sent out to get fabricated, not one I did at home by etching it myself. The board is for a Teensy LC, and breaks out most of the pins to screw terminals.
Once all the pieces are in place and connected up I switched to PCB view where you get to see what the actual printed circuit board will look like. […] It will output a whole bunch of files to the folder specified. Once I had this folder I made a ZIP file from it, and named it Teensy-BOB.zip and uploaded it to OSH Park.