Kurt in his boxing shorts
Go to the main menu. Hold down R2 + L2 and press Square twice, Triangle and Square.

Slow Motion
Play as Max, hold R2 and press Up, Up, Up, Up.

Multiple Characters
Pause game, hold L2 and R2, then up, down, right, left, x, triangle, circle, square, square + start.

Move camera
Pause, then hold L2 + R2 and press circle, x, circle, x.

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.

Invincibility
Pause during a game, hold L2, R2 and press Up, Up, Down, Down, Left, Left, Right, Right, Square, triangle, Square, triangle, Select, and Start.