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.  
  
 Result                      Code 
 --------------------------------
 God mode                    /god 
 No clipping mode            /noclip  
 Opponents ignore character  /notarget  
 All weapons and ammo        impulse 101  
 Map select                  /map   
 Third person view           /thirdperson  
 Adjust gravity              sv_gravity   
 Obtain item                 /give   
 
 Hidden door:
 Enable cheat mode, then activate the /map redE3M5 code to 
 advance to episode 3, mission 5. Locate the door that does 
 not have a handle in that level. Enter the /noclip code and 
 walk through the door to find another door covered with 
 smiley faces.