Cheat List:
Most cheats from Half-Life already work with the Blue Shift expansion, but here're some of the most important ones:

god - enables you to become invincible
noclip - enables you to walk through stuff
notarget - enemies cannot see you
impulse 101 - gain all guns and max ammo
sv_gravity # - changes the gravity setting (default # is 800)
sv_accelerate # - set acceleration of player (default is # is 10)

For these codes bring up the console window with the TILDE key.

Tip: Combat Boost:
When you are transported to the alien world, find the round puddles and stand in them. In many encounters, you can save your butt by standing in them and fighting. They supply infinite health and when you are injured they will slowly raise your health, therefore, you will not take as much damage. (Note there is one encounter that will almost require you to do this unless you are an exelent player and/or you have full suit health)

Turn off the Flashlight:
To turn off the flashlight, pause the game and hit your flashlight key.