Press the console key ` and type "cht_bEnableCheats = 3" to enable the developer cheats. Note that console commands are case sensitive. You can access the cheats through the menu (Press ESC then press F3) or by typing them into the console. (Type "cht" into the console and press TAB for a list of cheats.) Doing this however will invalidate your achievements and leaderboard score gained in that session. Type "cht_bEnableCheats = 0" in the console to disable the cheats for the next session.

Effect	Code
Auto aim cheat.	cht_bAutoAim
Display all the messages in NETRICSA.	cht_bShowAllNetricsaMessages
Enemies can't see you.	cht_bInvisible
Fly around and pass through walls.	cht_bGhost
Fly around.	cht_bFly
Gives a specified amount of ammo for a specified weapon.	chtGiveAmmo()
Gives a specified amount of armor.	ch_slGiveArmor
Gives a specified amount of health.	cht_slGiveHealth
Gives a specified weapon.	chtGiveWeapon()
Gives all available weapons and maximum ammo.	cht_bGiveAll
God mode.	cht_bGod
Greatly increased speed.	cht_bTurbo
Infinite Ammo.	cht_bInfiniteAmmo
Kill all enemies in line of sight except for bosses.	cht_bKillAll
Kill the current boss.	cht_bKillBoss
Unlocks Mental Mode. (Also available through the custom difficulty menu or by beating the game on Serious)	cht_bUnlockMentalMode