Load the game with the "hl.exe -dev -toconsole" command line. Then while playing a game, press ~ to display the console and type "sv_cheats 1" to enable cheat mode. Then, press ~ and enter one of the following codes to activate the corresponding cheat function:
Result
Cheat Code
God mode /god
All weapons with full ammo /impulse 101
No clipping mode /noclip
Opponents ignore character /notarget
Third person view /thirdperson
Adjust gravity sv_gravity [-999 - 999999]
Obtain item /give [item name]
Map select /map [map name]