Various Cheats
     Start  the  game  with  the  command-line  "-dev -toconsole" (eg,
hl.exe  -dev  -toconsole). If the orange text doesn't appear, press ~.
During  gameplay,  press  ~ and type sv_cheats 1 to enable cheat mode.
Now, press ~ and enter any of the following "Codes":

Code                            Effect
/god                            God Mode
/noclip                         No Clipping
/notarget                       Opponents Ignore Character
impulse 101                     All Weapons and Ammo
/map (map name)                 Map Select
/thirdperson                    Third-Person View
sv_gravity ####                 Adjust Gravity to -999 to 999999
/give (item name)               Obtain Item