I'm very lucky to have a $DAYJOB as a Free Software Developer working for Linaro. I spend most of my time working on QEMU which is a open source virtual machine monitor (VMM) and emulator. I have also done some work on the Linux kernel, one of the largest most collaborative projects in the world.
Pretty much everything I write both professionally and personally is licensed under free and open terms, usually the GNU GPL. Aside from a large number of quick hacks and scripts I also maintain a browser plugin called Edit with Emacs which allows browsers (Chrome and Firefox) to hand of editing of text boxes to your favourite editor. I have also authored a number of Emacs extensions which can be found on MELPA.