The Notorious P.I.G. is a modern classic. You can find these guys at the farmer's market, outside the Atlas and other random places around town. I usually purchase from them whenever I can smell their delicious cart. If you can't find them, their twitter account usually has their current location: <a href="/redir?url=http%3A%2F%2Ftwitter.com%2Fthehappypig&s=c1fcf75433237b35dcc2e692b385b0a8f2c8d751b7c06d85d80e34613bd395e8" target="_blank" rel="nofollow">http://twitter.com/theha…</a>
It's a little pricier than what I'm used to paying for food from a cart, but they also use top notch local ingredients so I guess I can't complain.
I recently moved up to Milwaukee; I hope they open a franchise cart near my current location!