So the creators of the game Strategic Domination needed a way to tell people about it, but didn't have the budget for a billboard. Then they noticed all the homeless people downtown and came up with an idea: Staple a mini-billboard to the hobo's own cardboard sign, making both more noticeable.
They call this concept "Bumvertising." (A note on dialect differences: "Bum" in American English is a homeless person, but in British English is a butt. We mean the first kind here. A "bumvertisement" in the UK sense would probably violate several of their laws about public nudity. All further "bum" references in this article are in the US sense.)
I love the win-win aspect of this project. The advertiser gets something about a quarter as effective as a billboard for less than 1/100th the price. The bum gets additional income for something he or she was doing anyway. While the added income isn't quite enough to lift the bum into a studio-apartment, it is enough for clothing-replacement, and maybe a better meal. Maybe that's enough to finally go out and find a real job, but probably not. People don't live in the street for fun, mostly.
The only downsides I can think of are: 1, enforcement. How does the advertiser know that the bum stayed on his corner, and 2, chance that another bum will mug the bum that accepts the advertising. (I don't want the advertising sign to mean "Hey, I've got money, feel free to rob me.")