Our neighbors keep chickens (got rid of the rooster, thank gawd), and I know of urban bee keepers as well. Lots of options if you keep your profile small, even in a crowded urban environment.
My wife keeps talking about getting the neighbors together to tear down the fences that separate our back yards, and turning the space into a functional urban farm with crops, animals, bees, fruit trees, etc. The average yard in our neck of the woods is about 40 feet by 25 feet, so on our own there isn't much room to do stuff. But if we get 6 or 8 neighbors to combine our space, we could grow and share quite a bit. I doubt the neighbors will go for it, but if society ever starts to collapse it might be worth pushing the idea. Problem is, we barely know any of our neighbors at all. Nice idea, tho.
you don't have to tear gown the fences, just put in plenty of gates!
(John Seyemore, The self sustained Life and how to live it, it's been on my night stand for a long time now....he also has a gardening book out. Good ideas on how to make small properties work hard to feed the family)
http://www.amazon.com/John-Seymour/e/B001ILFL04