GTA V Very Easy Money Exploit (PS3)

In the ocean near the north side of the map, you can check underwater to find a crashed airplane. When the airplane crashed, a briefcase full of money (around $12,000) also found its way to the sea bed. You can swim to find the wreckage and the briefcase. After you collect that briefcase, you can then immediately switch to another character and then switch back to the one who was located near the briefcase. When you do, the briefcase will have reappeared. The character who is underwater will collect the case again and his supply of air will return. You can repeatedly use this trick to quickly amass a small fortune.

To make this trick even more useful, you can set things up so that you're switching repeatedly between two characters who are both near a briefcase found in a separate area (they can't both be near the briefcase that is underwater because one of the two characters will soon drown from a lack of oxygen). That way, you're rapidly building a fortune for both characters, instead of for one of them at a time.