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 mod has a lot of potential. But I feel like it didn't live up to what it sounded like. The maps aren't the greatest and the AI is horrible, but it just didn't get enough attention for them to continue. They completely stopped making updates for it, and only now are they updating this page, but only because it got added to Desura. I give this a 6/10. Would be amazing if it was still in dev.