A green roof can lower the heating and cooling costs for your building. A green roof is a layered system with a soil medium, and carefully selected low-maintenance plants. You can grow herbs, flowers and create a relaxing space for residents. The City of New York is implementing partial tax abatements to help you recoup some of the cost of constructing a green roof.
http://www.nyc.gov/html/dob/html/sustainability/green_roof_faq.shtml