Slow Motion: Play as Max, hold R2 and press Up, Up, Up, Up
Kurt in boxers: Hold L2 + R2, then square, square, triangle, square at main menu
Multiple Characters: Pause game, hold L2 and R2, then up, down, right, left, x, triangle, circle, square, square + start
Weird camera views: Pause, then hold L2 + R2, and press x, circle, x, circle
Gassy sounds from Hawkins: Play as Dr. Hawkins, then press L2 + R2 + Left + X.
Pause gameplay, hold L2 + R2 and press Up, Up, Down, Down, Left, Left, Right, Right, Square, Triangle, Square, Triangle, Select.