Portal: Still Alive, but instead of it being a mod on your Steam library, it's a map pack you can find in Bonus Maps.
This is the second, more complete version of the pack. Installation instructions in the description.
This is the second, more complete version of the pack. To install, make a folder in your custom folder (steamapps/common/Portal/portal/custom, folder can be any name). Then put the contents of the zip in your folder.