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.
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. Installation instructions below (click the "+read more" link).
How to install:
WON version (up to 184.108.40.206):
1. Close your game.
2. Extract the archive you've downloaded to your Half-Life directory (e.g. C:\Sierra\Half-Life) so the end result should look like this - C:\Sierra\Half-Life\zombie\ (there should be no C:\Sierra\Half-Life\zombie\zombie folder or C:\Sierra\zombie\ folder)
3. Start Half-Life, go to "Custom game" and select Half-Life: Zombie Edition from the list.
4. If you can't find it on the list, try creating a shortcut to your hl.exe application. After you've created it, add " -game zombie" after filename in shortcut options.
Steam version (220.127.116.11):
1. Close your game and Steam.
2. Extract the archive to your Half-Life directory (e.g. C:\Steam\steamapps\yourusername\half-life, so for me it'd be C:\Steam\steamapps\shooter__andy\half-life) so it should be like this - C:\Steam\steamapps\yourusername\half-life\zombie\ (there should be no C:\Steam\steamapps\yourusername\half-life\zombie\zombie folder orSteam\steamapps\yourusername\zombie\ folder)
3. Start your Steam client again.
4. Now you can either make it so you start Zombie Edition when you start Half-Life, or you can create a shortcut to the mod directly without changing the original game:
4.1. For the first option, go to your games library tab on Steam, find Half-Life, right-click on it in the list, select "Properties" and then press "Set Launch Options" button. Add "-game zombie" without quotation marks.
4.2. For the second option, create a shortcut to your hl.exe application. After you've created it, add " -game zombie". Add it on Steam ("LIBRARY" -> "Add a Game" -> "Add a Non-Steam Game", select your shortcut) and do what 4.1. says for this new game.