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.
Unpack this (.exe installer will be avaible later) to: YourDisk:\YourHalf-LifeFolder\ (for WON version) YourDisk:\Steam\steamapps\YourUserName\Half-Life\ (for Steam version) Don't forget to apply hotfix to 1.01! You can get ot in the downloads section here.