I spotted an activist campaign on Twitter today whereby you click a link and your Twitter avatar turns green or you can get a green ribbon added to it in order for you to show your support for democracy in Iran.
You can check it out at http://helpiranelection.com/
I thought it was an interesting application.
Unfortunately I am too time poor today to find other examples of this type of application but I am sure that you can find other examples of this type of thing elsewhere.
Apps like this can provide opportunities for markateers to create advocacy.