Uncommon Ground, an innovative restaurant in Chicago, took their innovation with food to a new level - the roof. The nation's first organically certified rooftop farm it offers vegetables, beehives, and solar panels. Throw in an educational outreach component to a local school and the community at large, and you want to climb up on your own roof to see what the possibilities might be.
For more information on how they did it - from materials to contractors - check out the Uncommon Ground Green Fact Sheet. You can also follow the progress of things at the farm at their blog - Eat This, Grow That - and get loads of other tips and ideas for your own urban farm or garden.
Rooftop gardens, whether for growing vegetables or simply having a green roof, offer numerous benefits. Lower temperatures, better air quality, and sometimes the roof is the best place to grow those sun-loving varieties that otherwise might not be an option. It is also a great place to create green space when surrounded by city.