You can activate the command console by pressing the ~ or ¬ key on your keyboard (Whichever is next to the number 1). Please note that not every command, item or character is listed here, as the list is very long, try experimenting for yourself. :)
sv_cheats 1 - Enables cheats. god - Makes you invincible. notarget - Enemies will be unable to target you. noclip - Allows you to walk through doors, walls etc. cg_thirdperson 1 - Switches to third person view.
give (item) - Replace (item) with one of the following:
Thanks to Black Ice
npc spawn (name) - Replace (name) with one of the following: