It depends....if there is no identification and simply cash.
If it is a single bill up to $100, I probably would take it, give 10% to charity and keep the rest. WHy do I say a single bill especially in the middle of a busy street? Because most likely, the person will not notice it if it falls out of us pocket or something...maybe I'm wrong with this assumption..IT's possible if it were a large bill I would wait around, but futilly.
If it is a whole wad of dough...and it is on a sidewalk, I would pick it up, hold on to it on that spot and hope that someone will come by frantically retracing his steps. If i'm in a cab, I would call the cab company in hope they trace the person...
If nobody claims it after a week or so, I would do the same, 10% to charity, the rest kept. I definately wouldn't leave it around on teh sidewalk for the simple reason that we live in an evil society and most people will just take it and not at least try to do the right thing.