Select "Options", then the "Keyboard" tab, then "Advanced". Place a check "Enable Developer's Console" then click "OK" or "Apply". Press ~ during game play or a the main menu to display the console window. Type sv_cheats 1 to enable cheat mode.

Effect	Code
Bind command to key	bind [key] "[command]".
Deactivate cheat mode	sv_cheats 0.
First person view	firstperson.
Refill ammo regardless of inventory and Speed Bonus/Malus	givecurrentammo.
Set speed; 1 is default	host_timescale [number].
Third person view with alternate controls	thirdperson_platformer 1.
Third person view	thirdperson.
Toggle unlimited Run/PsiForce	sv_infinite_aux_power [0] or [1].

Note: You can also bind any cheat code to a key, for example bind x "sv_cheats 1" to execute the svcheats 1 code by pressing X.