Half-Life cheats and cheat codes for Playstation 2

Enter the following button combinations at the cheat screen:

Invincibility:
Press Left, Square, Up, Triangle, Right, Circle, Down, X

Unlimited ammunition:
Press Down, X, Left, Circle, Down, X, Left, Circle

Slow motion mode:
Press Right, Square, Up, Triangle, Right, Square, Up, Triangle

Invisibility:
Press Left, Square, Right, Circle, Left, Square, Right, Circle

Alien mode:
Press Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle

Xen gravity:
Press Up, Triangle, Down, X, Up, Triangle, Down, X

Unlock Alien mode - Secret 10th mission:
Finish all 9 Decay levels with an "A" rank. 


Unlock Bonus levels:
Go to the cheat screen. 
Press Triangle, Square, Left, Right, Circle, X, Up, Down 
Select the "Extras" menu in the options screen to load new levels found 
from other discs, such as the Official US PlayStation Magazine Issue 57 
(June 2002) demo disc. 

Unlock Alien mode - Secret 10th mission:
Finish all 9 Decay levels with an "A" rank. 

Save ammo:
Use guns to save ammo for other weapons. Kill the enemy  with 
the indicated numer of pistol shoots:

Crabs: 2 shots or 1 crowbar hit
Dogs: 3 shots
Electric Aliens: 3 shots
Machinegun on stand: 5 shots
Soldiers: 5 shots
Spitters: 4 shots 
Trip Bomb: 1 shot
Zombies: 4 shots