Some folks like to grow vegetables and other plants in abandoned chunks of soil amid urban concrete. There are lots of online resources.
A Facebook group
http://www.facebook.com/group.php?gid=4605708492
which led me here
Based in London, these folks
http://www.guerrillagardening.org/
Which links me here:
Los Angeles!!!
http://www.laguerrillagardening.org/
Edmonton
http://edmontongg.blogspot.com/
Toronto
http://www.publicspace.ca/gardeners.htm
Brussels
http://brussels-farmer.blogspot.com/
Somewhere German?!
http://www.gruenewelle.org/forum.html