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 using new weapons and abilities. For further information please check this mod's Official Page.
After 1.5 years... It's back! Fixed some mapping bugs, added barnacles, NPC rushers, some alternative ways, scripts, steam icons, final speech (lame one, but I did everything I could) and final music by Toni Karengin.