Invincibility:
Enable this code on the Access Codes menu to become invulnerable - OTIS LOVES DREAMCAST

Level Passcodes:
Enter these Passcodes to unlock the corresponding level

Gordon goes to work Anomalous Materials
Gordon teaches physics Unforseen Consequences
Files and fear Office Complex
Soldiers visit Black Mesa "We've got hostiles"
Safe and Sound Blast Pit
Big is Beautiful Power Up
Trains and rockets On a rail
Fear finds Gordon Apprehension
Recycle or Die Residue Processing
Regression brings progress Quesionable Ethics
Combat and death Surface Tension
Soldiers eat tacos "Forget about Freeman!"
Scientists at work Lambda Core
Fear and Gravity Xen
Panic in Xen Interloper
Xenophobia and firepower Gornarch's Lair
Panic and die Nihilanth

See the Ending:
Input this at the Secrets section, by moving the three bars up and down until they spell out this "sentence"

Answers Begat Questions

Half Life - Blue Shift:
Input these in the "Secrets" section, in order to access the levels of Half Life : Blue Shift.

Barney goes to Work Insecurity
Red is Scary Duty Calls
Combat and Trains Captive Freight
Barney visits Xen Focal Point
Power is Progress Power Struggle
Physics is Beautiful A Leap Of Faith

Slow motion code:
By completing the training mission you will receive this code. Under Options select Enter Codes, using the three word wheels make them show the proper phrase and hit A to verify.

Actions Ignore Silence

Unlimited Ammo:
In the options screen under codes, enter the following, push a to confirm then go into trigger codes and turn it on. It told me this after I beat the blue shift game.

DREAMCAST GIVES FIREPOWER