Access the console, type in "npc_create npc_" and replace the name with one of these:

Code	Effect
barnacle	Spawns a barnacle.
turret_ceiling	Spawns a Ceiling Turret.
citizen	Spawns a City 17 Citizen.
combinedropship	Spawns a Combine Dropship
helicopter	Spawns a Combine Helicopter
metropolice	Spawns a Combine Metrocop
cscanner	Spawns a Combine Scanner.
combine_s	Spawns a Combine Soldier.
crabsynth	Spawns a Crab Synth
crow	Spawns a Crow
headcrab_fast	Spawns a Fast Headcrab
fastzombie	Spawns a Fast Zombie.
turret_floor	Spawns a Floor Turret.
launcher	Spawns a Headcrab Launcher
headcrab	Spawns a Headcrab.
ichthyosaur	Spawns a Ichthyosaur.
manhack	Spawns a Manhack
mortarsynth	Spawns a Mortar Synth
pigeon	Spawns a Pigeon.
headcrab_poison	Spawns a Poison Headcrab
headcrab_black	Spawns a Poison Headcrab
poisonzombie	Spawns a Poison Zombie.
rollermine	Spawns a Rollermine.
seagull	Spawns a Seagull
stalker	Spawns a Stalker.
strider	Spawns a Strider.
vortigaunt	Spawns a Vortigaunt.
zombie_torso	Spawns a Zombie without legs.
zombie	Spawns a Zombie.
zombine	Spawns a Zombine (Combine Zombie)
alyx	Spawns Alyx.
antlionguard	Spawns an Antlion Guard.
antlion	Spawns an Antlion.
barney	Spawns Barney.
breen	Spawns Breen.
combinegunship	Spawns Combine Gunship
dog	Spawns Dog.
eli	Spawns Eli
monk	Spawns Father Gregori
gman	Spawns Gman
kleiner	Spawns Kleiner
mossman	Spawns Mossman