I had a guy ask me for $2 once; but before I could say no he offered me a screwdriver. he evidently needed two bucks and was trying to sell whatever was lying around in his car or something. So I took his offer... that screwdriver turned out to be one hell of a tool! Reversible phillips/flat, plus six removable tips all contained inside the handle. Best $2 I ever spent.
But aside from him, I never give them money but I have dropped off sacks of warm food from nearby drive-throughs. At least they eat that, instead of spending it on beer.