Foreign currency, usually printed in bright colors with funny pictures of people that no one has ever heard of. Called Monopoly Money or Funny Money because it looks and feels like some fake cash you get with a board game.
Monopoly Money is often spent recklessly because it looks fake, therefore you're not spending "real" money.
British Pound
New Zealand Dollars
Canadian Dollar
Japanese Yen
Euro
Australlian Dollar
Switzerland Francs
South Africa Rand
Mexico Pesos
Russia Rubles
Saudi Arabia Riyals
etc