During play, press F2 bring up the console. Then type one of the following codes
and press Enter.

invincible 1       God mode 
invincible 0       Disable God mode 
health             Full health   
invisible 1        Invisibility  
invisible 0        Disable invisibility 
ammo               Full ammunition   
fly                Flight mode  
ghost              No clipping mode   
walk               Disable flight and no clipping modes   
stealth            View stealth amount   
playersonly 1      Freeze enemies   
playersonly 0      Unfreeze enemies 
killpawns          No enemies   
summon <item>      Spawn indicated item      

Item names:
Use one of the following with the summon code:

echeloningredient.ecamerajammer
echeloningredient.echemflare
echeloningredient.edisposablepick
echeloningredient.ediversioncamera
echeloningredient.ef2000
echeloningredient.eflare
echeloningredient.efraggrenade
echeloningredient.elasermic
echeloningredient.elockpick
echeloningredient.emedkit
echeloningredient.erappellingobject
echeloningredient.eringairfoilround
echeloningredient.esmokegrenade
echeloningredient.estickycamera 
echeloningredient.estickyshocker
echeloningredient.ethermalgoggle
echeloningredient.ewallmine 
  

Thanks to Revolution readers |Stealth|, Speedfight, Andreas Aabrandt, JMN,
Viperomega1 and FateoMcSkippy!