The ancient legal principle of usufruct broadly dictates that private property can be used for the public good so long as it's not damaged in the process. This is of note to the urban forager, as it suggests that fruit and other plant foods grown on private land can be harvested by passersby. A stricter and far less hazardous foodie interpretation of usufruct means that ripe citrus tree, whose trunk meets the soil inside your neighbor's yard but whose laden branches overhang the sidewalk, can be shorn of a few bits of fruit so long as you don't harm the tree or any other property in so doing, or abuse the privilege; that is, take only as much as you can consume.
In some countries and cultures, usufruct is codified into law. In others, like the United States, it's regarded as a threat to private ownership and a salvo into the realm of collectivization. If you don't wish to test your neighbor's understanding of legal arcanum (or his adherence to the Second Amendment), keep your hungry hands on public land. There's more than enough goodies to go around if you're willing to walk and gather.
In Pasadena's Memorial Park, a goliath avocado tree sheds its fruit in summer from branches some 30 feet up, in case you were wondering what that dense thud was as you were taking lunch at one of the park's battered picnic tables. Rosemary and lavender sprout wild across the city, and those handy with brine can find olives to cure in stands on both the West- and Eastside. But it take a patient eye to spot the elusive wild Overland Avenue banana, resembling a miniature version of the corporate Musa acuminata shipped green by the ton from the Pirate Latitudes and consumed annually at the rate of 25 pounds per capita. Left to ripen on the stalk and plucked when they display a few spots of brown, these squat bananas are a secret hiding in (almost) plain sight--mildly starchy, a lot sweet and free to peel.