Cheat mode
Load game with the command line: hl.exe -dev -console. Then, press ~ if the orange text on the screen does not disappear. During gameplay, press ~ to display the text prompt and type in sv_cheats 1. Press ~ and enter one of the following console commands to activate the corresponding cheat function.

Code - Effect:

/god - God mode.
sv_gravity <-999 - 999999> - Adjust gravity
impulse 101 - All weapons and ammo
developer 1 - Developer mode on
developer 2 - Verbose developer mode
developer 0 - Developer mode off
kill - Suicide
/map - Map select1
/noclip - No clipping mode
/give - Obtain item2
/notarget - Opponents ignore character
/thirdperson - Third person view
/firstperson - Return to first person view
+reload - Auto-reload enabled
-reload - Auto-reload disabled
lambert -1.0001 - See things brightly without flashlight
impulse 101 - HEV suit, weapons, and ammo
impulse 102 - Spawns gibs and blood
impulse 103 - Monster stats, while looking at monster
impulse 104 - List global entities
impulse 105 - Silent player
impulse 106 - Model/sprite stats, while looking at target
impulse 107 - Texture name, while looking at object
impulse 195 - AI node information
impulse 196 - AI node information
impulse 197 - AI node information
impulse 199 - AI node information
impulse 203 - Delete monster or NPC from world

Delayed death
Enable the impulse 101, impulse 105, and notarget codes and get the Magnum. Walk up to a scientist from behind, then shoot him in the back of the head. Certain ones will make a noise, stand for about five to ten seconds, then fall and die.

Talk to dead security guard
Get the HEV suit and enable the impulse 101, impulse 105, and notarget codes. Reactivate the cheat screen, and keep using the impulse 101 code until your HEV suit is full. Choose the high explosive (press 5 twice) and return to the lobby in Sector C Test Labs, where the security officer is on the computer. Go behind the desk and throw a high explosive under the desk. Walk anywhere you want, but not past the loading point, then detonate it. Walk back and the security officer should be on the desk. Talk to him.