http://www.change.org/
They do an awful lot of good work - they've helped free a number of wrongly-detained political prisoners, prevented a lot of environmental destruction, and are helping with the progress of womens' and minority rights in a lot of areas. I think they've been instrumental in the recent law change in Saudi Arabia which has given women the vote there. Joining costs nothing, and you can select whichever petitions you want to sign. It doesn't seem like doing much, but every signature on any petition helps. Just thought people here might be interested.