Press L + R simultaneously to bring up a cheat interface where you can enter these codes:
MIDAS - Unlocks 2 Player mode
FREEALL - All items cost nothing (does not apply to services)
PARTY M - Unlocks Party Motel in 2 Player Mode
SIMS - Unlocks "Play The Sims" Mode
FISH EYE - Unlocks first person view (press Black to change view)