A really great site is loesje.org (english) and loesje.nl (dutch).
From their english site :
Loesje is a Dutch girl with friends and connections all over the world. She wants people to get active, think and react on everything that happens around us. She believes that everyone has something to say about the world we live in. Loesje spreads her ideas and thoughts through posters.
Anyone is free to join in and help her in her work!