Certain enemies in the Half-Life series of video games are named zombies, although they are not of the traditional type described in the above writeups. Technically speaking, they aren't even dead. I suggest you check out headcrabs if you'd like to know more about Half-Life's zombies.
Headcrabs, the tiny alien freaks from Xen, have the instinctive ability to latch onto the nervous systems of sentient species. By latching on this way, a headrab will gain total control of the host's body and eventually mutate into a part-alien zombie.
The zombie will share the form of whatever it was before being assimilated, but it will quickly develop common characteristic mutations. The chest of the host splits open straight through the rib cage and the headcrab begins to merge into the host's skin, which takes on a green tinge. The host's fingers extend into sharp and powerful Nightmare on Elm Street-esque claws, which it uses to bash and rend prey. The zombie then uses these claws to pick chunks of flesh off of the kill, and stuff them into its chest-gash, which serves as a grotesque parody of a mouth.
The zombie is a meat suit, shambling around without much speed, with the headcrab in full control. Headcrabs aren't too bright, luckily, so the zombie isn't much smarter.
The vertical maw is the source of the zombie's official name: The mawman. The general public had the tendency to call them zombies, however, because of their slow movement and severely limited intelligence, along with their somewhat resurrective origin. The name stuck, and now you're unlikely to hear the word 'mawman', even amoung die-hard fans.
Generally, zombies will stand around until they see prey, at which point they lurch after it, kill it, eat it, and then stand around some more. In order to compensate for their lack of speed, zombies will sometimes forcefully kick and toss objects at their prey. If a zombie is confined, it will go into a 'destruct mode', smashing everything in sight until it is free. This is often a sound tactic, as they are fully capable of clawing through a weak brick wall if the need arises.
When a zombie is killed, the headcrab may leap off of the useless dead body, if it has not yet begun to merge. Thus, the most efficient means of disposing with a zombie is to shoot it in the head, killing both the headcrab and the body at once. This is a good plan of action since zombies are resilient foes, and will continue to claw their way towards a target even if their legs are blown off. Fire is also an effective weapon: zombies are quite flammable.
If it manages to survive for around two days of feeding, the zombie will grow into a much faster, bulkier and more alien zombie, called a gonome. These enemies, featured in the expansion pack/sequel Half-Life: Opposing Force, can pull out their acidic stomach contents and toss them at you if you avoid their claws and jaws.
The zombie has been updated for the sequel of the Half-Lifes, Half-Life 2. It has has not yet been released, so updates will be added when it is.