Mo' money on the street

Today I found eleven bucks on the sidewalk. As opposed to last time, I didn't see who dropped it and I didn't see any likely suspects, so I stuffed it in my pocket.

What's the right thing to do? Leave it there in case someone decides to retrace their steps? Even though it's only eleven bucks, that might be all the cash someone had for the day (or until pay day). On the other hand, they were careless with their money. I'm not desperate for cash and could've just left it there, but if it blew into the gutter or something it would be wasted. Did I do the right thing? If not, what's the right thing to do?