I’ve just donated to the GNOME Foundation. Not much (but since I got back to studying for this year, my <marketspeak>revenue stream</marketspeak> has really slowed down), but I plan to donate more money, aside from my time as a developer, to the F/OSS projects that I use the most.
There are many F/OSS projects, out there, which asks for some support: from patches to documentation to translations; sometimes, even a little contribute if you find that project useful. Since many of these projects are free (as in beer, other than as in speach), a little donation might show some appreciacion for the wonderful jobs done by the people working on those projects.