Community Gardens Edit
Community gardens and urban farms are public open spaces which are devoted to the cultivation of plants – usually food, native or rare species – and managed by community groups.
Community gardens are usually managed using sustainable practices such as water conservation, worm farming and composting. The gardeners distribute the produce among the group and sometimes sell surplus.
They are usually initiated by a community group and supported by local councils, often with additional support from other agencies such as universities, state government agencies, local nurseries and other entities.
Community gardens are of benefit to individuals, community and the environment. Gardening provides some physical exercise and mental relaxation to practitioners, and result in fresh food. The gardens provide education and social activity, are usually created in abandones sites and greatly improve the aesthetics and safety of an area, and they provide a habitat for wildlife and clean the air.
Recent green lifestyle initiatives like the 100 mile diet  (about eating food that is locally produced to reduce transport costs and focus on freshness, seasonality and so on) and guerrilla gardening  (unofficial gardening acts to improve public space) are extensions of the community garden/urban farm movement.
Some groups which provide information/support/advice:
Australian City farms and Community Gardens Network 
Australian Community Foods 
In the United States:
American Community Garden Association