It's well worth reading the story of how Avaaz stopped Murdoch. It gives a bit of hope that other such corruption can also be stopped.
Murdoch Campaign Report
I think the ease of signing the petitions, ie just giving your email address, has helped a lot. Another one that makes it easy to support online campaigns is
Human Rights First. Just sign up with your email and receive brief messages from time to time with a link to the petition so you can add your voice. I always find the petitions fit with my sense of fair play and justice.