Half-Life: Decay

Half-Life: Decay is the third and final expansion for Half-Life. Developed by Gearbox Software and released with the PlayStation 2 version of Half-Life, Half-Life: Decay was a co-operative campaign that put the players in the roles of Dr. Gina Cross and Dr. Collette Green, two scientists who also worked at the Black Mesa Research Facility during the Black Mesa Incident.

On September 23, 2008, a group of Ukrainian programmers successfully ported Half-life: Decay to PC. It features online co-op gameplay and several other tweaks that were not in the PlayStation 2 version of the game. However, it's not available on Steam