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.

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.

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

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.

Saving ammunition
Do not waste precious pistol ammo on headcrabs. Use your crowbar, as it is more useful.

Glitch: Floating doctor in wheelchair
*** This trick requires a 2nd player. When playing a Decay game with another player, get to the Rift level. When the handicapped doctor tells you to follow him and opens the door, do as he says. He will get in the elevator. Block his path so he cannot get in the elevator, but the door is open. Next, back up slightly and let him enter. Get behind him and run forwards while the other player pushes you from behind. The doctor should start to go down, and the door has closed on you, but it is still open. Jump frequently and press Forward quickly. If you jump onto the safety bar where the top of the other door is, he will get to the bottom of the elevator. If you stand on the bar, and he gets to the bottom, then he will launch back up and will float in the air. You cannot kill him with a regular gun from below. You must shoot him with the bazooka or from above to kill him.

Xen gravity
Press Up, Triangle, Down, X, Up, Triangle, Down, X at the cheat screen.

Invincibility
Press Left, Square, Up, Triangle, Right, Circle, Down, X at the cheat screen.

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 1st 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.

Alien mode
Press Up, Triangle, Up, Triangle, Up, Triangle, Up, Triangle at the cheat screen.

Unlimited ammunition
Press Down, X, Left, Circle, Down, X, Left, Circle at the cheat screen.

Half-Life (PS2) cheats
# 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.

Half-Life (PS2) Game Shark Codes
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

How to Kill the Final Boss
To kill the final boss, you need to shoot the big yellow crystals which are located around the top of the big chamber. Then, when they are gone, use the things on the ground which make you bounce into the air and keep using them until you find yourself far above the Boss. Keep firing at his head until it opens. Once it is fully open. Use the bouncer things to get above him again, and shoot his brain. Repeat this and he WILL DIE.

P.S. Make sure you save as he starts to die as it is really funny to watch ROFL.
P.S.S When you are in the train with Gman after you kill the boss, he offers you a job. ACCEPT IT or you bassicly fail the Game.

How To Get To Gman In Room
To get into the room where you see Gman speaking with a scientist(before the accident), be an alien(Alien Mode) and you can break the glass. You can kill the scientist, but not Gman.

Awsome Weapon and Secret
This is one of the biggest secrets and most awsome weapons in Half Life.
All you have to do, is kill Gman when you see him. I know that you might fire every rocket laser bullet or bomb you have at him, butthe trick to killing him is to be quick enough to catch him. If you catch him, hit his breifcase on the handle (not Gmans hand or it wont work) with your pistol and his cas opens! Inside there is some paper, a gun, an ID, a poket pc and more notes. But, Gman turns around and starts to talk to you. When he is done, he pulls out a gun, and starts to count down from 10 until he kills you. Kill him first, blow up the body and you get the gun!Then gun fires headcrabs at people or animals and makes them fight for you. The gun also fires a laser that can go through any wall, allowing you to PROPERLY explore.