We've been buying cheap food off carts since the beginning of civilization, but over the past few years, there's been a quantum leap in the quality of the fare on the streets of many American cities. San Francisco, a city known for sophisticated palates and rigorous ethical standards, is home to Off the Grid, a flotilla of food trucks that roams the city, attracting musicians and crowds wherever it alights. I paid a visit to the event at Fort Mason, on San Francisco Bay, to see what the future of cheap street eats tastes like.