Money holds the power to shape the flow of games - from single player games to MMOs. With every game we make, we are designing currency. Sometimes the currency is simply points (or points in a more colorful guise). Other times it is a means of drawing the player towards challenges - collect x widgets and you can continue. In a third case the player collects money to gain power directly or indirectly: Direct mechanisms being things like Mario's 100 coins for extra lives or experience points to earn level-ups, and indirect ones being shopping and bartering.
One thing unifies those examples: The game has absolute control over the money supply. And in many games, it tends towards an initial scarcity that later collapses into an abundance of wealth; the player starts off weak, and has to pick opportunities carefuly, but inevitably progresses in skill or power, or finds loopholes in the system, and legitimately or not, they collect hundreds of extra lives, store thousands of pieces of equipment, boost their character's abilities to god-like ratings.