Today I put my money where my Free Software touting mouth is. That number, all 4 glorious digits, is my membership number for the Free Software Foundation. This is the organisation founded by Richard Stallman in 1985 to promote and support Free Software.
As the copyright holder of the copyleft GPL (GNU General Public License), it is responsible for legally enforcing transgressions of the GPL. It is also campaigns on a range of issues, from open standards, DRM through to critical software to support.
Plus, you get an operating system (free, of course) on a membership card! How amazing is that? Not that I'm swayed by such gimmicks, because it really is about practical support for what you believe in. In my case this is Free Software, but whatever your cause, dig out your wallet for 2010!