Guerrilla gardening is about transforming neglected urban spaces by illicitly working the soil and adding plants and flowers to reclaim them back into the public domain. There are many networks of guerrilla gardeners in many cities throughout the world, in fact there is a good chance one or two already exist where you live. With a little research you may be able to track down one of these groups and join in their activities, or alternatively start up your own local guerilla gardening team.
One of the most well known guerrilla gardening successes was begun by Liz Christy in New York City. To find out more, visit the Liz Christy Community Garden website, and if that inspires you, learn how to become a guerrilla gardener by following the link below.