A random guy who follows you around and every time you do something worth getting paid for but no one else will pay you, he will. Originated in GTA: San Andreas under the pseudonym "insane stunt bonus," but has since been seen in Israeli airports and other unexpected locales.
I just drove my car off a cliff, did a double flip, and landed right side up, so Random Money Guy gave me $22!
I just drove my motorcycle off a building, flipped 3 times, did a quadruple corkscrew, and landed on the roof of a truck on my feet, so Random Money Guy gave me $136!
I just lost all my luggage on the flight, so Random Money Guy gave me $75!