One of the organizations I volunteer for is Debian. This is a Linux distribution done as totally free (as in liberty, as well as beer) software. (The BSD and Hurd efforts haven't yet produced a stable release.) No one is paid by the Debian organization, but some people are allowed to work on Debian on company time. I am one of the thousand or so "Debian Developers", but many other people contribute to the release as well.
The main thing I've been doing for Debian lately is despamming the Bug Tracking System. Each bug has half a dozen email addresses, and the addresses can be found on the web and in mailing lists. With tens of thousand of valid email addresses, the BTS gets a lot of spam. At one point it was frequently over a gigabyte of spam a day caught by our filters. Now we have better pre-delivery filters, so less spam gets to that point. Anything that makes it past the filters (mostly a tuned spamassassin) gets delivered to the bug, and I post-moderate ones with questionable scores. Such moderated messages get fed back into the baysian filters, and on occasion I add more spamassassin rules based on what is getting through.
Debian is an international group that mostly communicates via email and IRC. I sometimes answer user questions in #debian on irc.oftc.net.
Debian has an annual conference, DebConf. I've managed to make it to five out of the last six, including New York City this year, the first time it's been in the US.