/pet as you were -- Makes pet forget all other commands and clears their aggro list.
/pet attack -- Pet will attack targetted creature
/pet kill -- Same as pet attack.
/pet back off -- Tells pet to quit attacking target. Note that if a monster attacks either you or the pet, the pet will start attacking again.
/pet follow me -- Pet will start following you (cancels any other guard commands)
/pet get lost - pet suicides
/pet guard here -- Makes your pet stand in one one location and attack any NPC that comes near, including guards. He will return to his original location after the fight.
/pet guard me -- Pet follows you around and attacks any NPC that comes near you.
/pet who leader -- Pet says "XXX is my leader." Used pretty much only by Enchanters and others who can charm critters.
/pet report health - Your pet will tell you what percentage of HP it has left. Useful, since you don't have to target your pet to find out how close to death it is.
/pet sit down -- Causes pet to sit in one place and rest much like you would sitting down
/pet stand up -- If your pet is sitting, he will stand up. Personally, I just use /pet follow instead of this.
/pet target -- Targets your pet. (also works if you hit F1 twice)
/pet taunt -- Your pet will taunt its targets.
/pet no taunt -- Your pet will not taunt its targets.