Cheat mode:

Load game with the command line: 
hl.exe -dev -toconsole. Then, press ~ if 
the orange text on the screen does not 
disappear. During gameplay, press ~ to
display the text prompt and type in 
sv_cheats 1. Press ~ and enter one of the
following console commands to activate the
corresponding cheat function. 

/god-God mode. 
/noclip-No clipping mode 
/notarget-Opponents ignore character 
impulse 101-All weapons and ammo 
/map [map name]-Map select 
/thirdperson-Third person view 
sv_gravity [-999 - 999999]-Adjust gravity
/give [item name]-Obtain item