Zombie Edition is a mod based on original half-life concept, but you will play as a headcrab and, what is more interesting, as a zombie. Zombies can mutate to change their characteristics into three types and every type will have at least six upgrades including new weapons and abilities. For further information please check this mod's Official Page.
This review may contain spoilers
Out of all the enemies Half-Life and it's expansion packs threw at you, The most common had to be those Headcrabs and Zombies/Mawmen. The stuff of nightmares they were, And you pretty much slaughtered a large amount of them in your quest to escape black mesa.
Half-Life:Zombie Edition puts you in the skin of a headcrab who can latch onto and turn humans into Zombies, in addition to the normal zombie there are two new types:Rusher and Bruiser. To get these upgrades you need to "Pay Money".
The mod can get hard at various times due to the headcrab/zombie having a slow movement speed (though the headcrab's leap can undo that). Security Guards, Scientists, even Human grunts are fair game for zombification.