The Friends of the Earth Nederlands branch has gone around the big cities of the Nederlands illegally planting trees to highlight the amount of illegal logging that goes on around the world. There is a brilliant picture here of a tree planted in the middle of a outside paved shopping area.
There is also an organisation set up to promote Guerrilla Gardening. It is for people who go around illegally planting gardens in public areas. I am way too amused by the concept of guerrilla gardening. I have visions of people in ninja gear creeping out in the dead of night with spades and compost.