- If you're engaged with the FOSS community, and contribute in any way, even if it is testing or translations, you should brag about it in your C.V.
- If you're not engaged with the FOSS community, and looking for a hobby (which doesn't necessary takes a lot of time), you should look for a piece of software you like and use, be it a torrent client, chat client, the tiniest usability app - and see what you can do to improve it. Maybe start by reporting bugs and/or try to fix them, or simply add a translation to your native language.
Recently it seems employers are getting even more ways of telling how good programmer you are, without even asking you questions or bringing you to an interview. Take for example Trollim, which ultimately allows you to get a rank which says just how good you are. Or take Google Code Jam, which doesn't "rank" you, but says something about you if you participate and advance. Specifying to your would-be-employer that you have participated in such activities, shows something about you and your passion to the profession. Think about it.