For me it depends on who's money it is, and who needs it more. If it's the corporation's money I'd take it as "karmic reward" and invest it wisely. However, if it belonged to an individual person I'd be inclined to hunt them down and return the money.
I'm all about honesty; it is the best policy. Given this situation it's more of a fortuitous opportunity. It's kinda like when you buy something and the cashier gives you more change than you expect; what do you do? Well, it depends on the cashier. If he/she was a bitch, then they warranted the error and the extra cash landed in my palm for a reason, otherwise I default to the honest response.