Load the game with the "hl.exe -dev -toconsole" command line and press ~ to display the console, then type "sv_cheats 1" to enable cheat mode. Press ~ and enter one of the following codes:
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]