Half life
Invincibility:
Press Left, Square, Up, Triangle, Right, Circle, Down, X at the cheat screen.
Unlimited ammunition:
Press Down, X, Left, Circle, Down, X, Left, Circle at the cheat screen.
Slow motion mode:
Press Right, Square, Up, Triangle, Right, Square, Up, Triangle at the cheat
screen.
Invisibility:
Press Left, Square, Right, Circle, Left, Square, Right, Circle at the cheat
screen.
Alien mode:
Press Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle at the cheat
screen.
Xen gravity:
Press Up, Triangle, Down, X, Up, Triangle, Down, X at the cheat screen.
Hint: Starting tips:
After the "accident", you will discover a scientist giving CPR
to a security guard. Wait 15 to 20 seconds to see the outcome, which usually
results in the security guard being revived. He is not useful at this time,
but return to him after getting the crowbar. Kill him with the crowbar so
he can not fire back to get his gun and ammunition.
Soon after this, you will ride an elevator upward. Have the gun ready. Shoot the monster attacking the security guard; a couple of shots should do it. This security guard can cover you until you reach the main entrance area.
One important tactic in the early game is to use your "allies" to your advantage. When you no longer require the services of a security guard, kill him and take his ammunition. A scientist is also useful in drawing enemy attack. Be careful not to harm an ally when another security guard is present, as he will attack you. Also be aware that monsters can sense your allies; and even if you crawl, monsters can hear your allies' walk.
The beginning of the game also has the following glitch. There is a scientist sitting in front of a damaged instrument panel, in the area of the first scientist who opened a door for you. This scientist is invincible. You can shoot or beat him with a crowbar, and all he will do is whine. This also saturates the area with a large amount of blood.
Hint: Saving ammunition:
Do not waste precious pistol ammo on headcrabs. Use your crowbar, as it
is more useful.
Hint: Using the pistol:
Use the following trick to save ammo for all guns other than pistol. Most
creatures or persons can be killed with five or less direct shots of the
pistol.
Crabs: 2 shots or 1 crowbar hit
Zombies: 4 shots
Electric Aliens: 3 shots
Trip Bomb: 1 shot
Machinegun on stand: 5 shots
Soldiers: 5 shots
Dogs: 3 shots
Spitters: 4 shots
Hint: Using guards:
If you have a guard, get him close enough and he will take care of almost
anything for you. The only time you ever need to fire is if the creature
is close to the guard.
Hint: Spawn Grunts:
Hint: Barney gets cut in half:
When you enter the room where the lasers are shooting out the walls, run
through. There should be a Barney (security officer) on the floor. Watch
carefully and you can see the laser go across and literally cut him in half.