Half-Life

# 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.
# Slow-motion mode

At the cheat menu, press Right, Square, Up, Triangle, Right, Square, Up, Triangle.
# 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.

Game Shark Codes

Master Code (Must Be On) EC8783A8 1431D934
Unlock Alien Mode Cheat 4CA573BA 1456F025
Unlock Infinite Ammo Cheat 4CA57372 1456F025
Unlock Invincibility Cheat 4CA5735A 1456F025
Unlock Invisibility Cheat 4CA5738A 1456F025
Unlock Slow-Motion Cheat 4CA573A2 1456F025
Unlock Xen Gravity Cheat 4CA57342 1456F025