For me this is sitting here with my laptop and typing away. Whether I'm blogging myself, signing petitions, writing to my MP, spreading the word on Twitter or Facebook or a half dozen other ways.
You can be part of any movement, and help anyway that you can - right from the comfort of your own sofa!
The UK Government has it's own ePetition site: http://epetitions.direct.gov.uk/
The Care2 Petition site is also full of ePetitions: http://www.thepetitionsite.com/
Avaaz is a campaigning network, you can like them on Facebook, follow them on Twitter or get email updates from them: http://en.avaaz.org/
You can help feed shelter cats and dogs by answering trivia questions:
The Greater Good Network has shops that support worthwhile causes and 'click a day' initiatives; everything from animal rescue to breast cancer:
Try out Microvolunteering site Help from Home, which positively invites you to participate in your pyjamas: http://helpfromhome.org/
Odds are whatever charity or cause is close to you they will have some way you can help from the comfort or your own sofa... so why not Google and see what you can do?!