Update May 2009: The price of aluminum and other metals has dropped precipitously, so you might want to hang onto your cans until the market recovers. If you live in a state that charges a deposit, though, cans can still save your can.
100,000 homeless can't be wrong! In states that require a deposit, 50 cans could bring you $2.50. As scrap, 32 cans make a pound, worth 70 cents.
When I need a little quick cash, I simply grab a little poundage from my cache of scrap aluminum cans and turn them into legal tender. Junk aluminum cans are everywhere, and just about any town has someone who'll pay cash for them. The trick is to get enough of them to make it worth your while, and find a way of managing them which keeps you from getting disgusted with the whole proposition.
Finding enough scrap cans is fairly easy. I scan parking lots and ditch lines. I keep a couple of plastic shopping bags handy to throw cans into. Most people you know send aluminum cans into their recycling bin. I have found that if you make a routine of collecting the cans from them for yourself, they are more than happy to hold them for you. Coworkers are also a good source of scrap cans.