Enter these on the "Extra" menu:
UP, UP, UP, R2, R2, UP - Extreme Ragdoll Physics
L2, RIGHT, CIRCLE, DOWN, R1, SELECT - Hippie Blood
LEFT, RIGHT, UP, DOWN, L3, SQUARE - Infinite Health
SELECT, R1, L1, SELECT, R3, L1 - Infinite Supplies
SQUARE, SQUARE, CIRCLE, CIRCLE, UP, DOWN - Low Gravity Ragdoll Physics
You can also obtain the ragdoll modifiers and hippie blood by beating the game once. A level select will pop up for clearing your first level and beating the game on any difficulty will open up the "Massacre" difficulty.