I don't participate much in this community because, to be honest I a) don't know as much about global warming as I do about other environmental issues and b) I don't have nearly enough time to do everything I want to!
Anyway, I'm dropping by now to mention the organization Greenpeace (www.greenpeace.org). You've probably heard of them. They deal with a huge variety of environmental issues, one of which is- of course- global warming. I haven't actually checked out any of their global warming actions yet, but they have a lot of quick & easy activism, online activism, and other kinds of activism projects on their site. You can also become a member and gain access to the forums (though I'm having some diffculty with them right now).