Halflife Cheat Codes for Playstation 2

Slow Motion:
If you look at the credits, (all the way to the bottom) You'll see the code: Right, Square, Up, Triangle, Right, Square, Up, Triangle

Invincibility
At the cheat menu, press Left, Square, Up, Triangle, Right, Circle, Down, X.

Infinite ammunition
At the cheat menu, press Down, X, Left, Circle, Down, X, Left, Circle.

Invisibility

At the cheat menu, press Left, Square, Right, Circle, Left, Square, Right, Circle.

Alien mode

At the cheat menu, press Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle.

Xen gravity
At the cheat menu, press Up, Triangle, Down, X, Up, Triangle, Down, X