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

cscanner
rollermine
poisonzombie
mossman
monk
metropolice
manhack
launcher
kleiner
ichthyosaur
headcrab_poison
headcrab_black
headcrab_fast
headcrab
helicopter
gman
fastzombie
eli
dog
pigeon
seagull
crow
combine_s
combinegunship
combinedropship
citizen
breen
barney
barnacle
antlion
alyx
antlionguard
stalker
strider
vortigaunt
zombie
zombie_torso