Here's an innovative online campaign from Swedish homelessness charity Stockholms Stadsmission, which ran this ad at Christmas.
It has invited websites and blogs, via Facebook and Twitter, to host its 'Homeless Banners'. The banners are downloadable on the campaign site, in several different formats. Once uploaded, over time, the banner will develop a warmer color the longer the site hosts it, and the more people that click on it. It also displays site's name and how long it has been hosting it. Clicking on a banner leads to the charity's donation page.