Source: PlanetHalfLife

Half-Life: Decay is coming to Half-Life 2. For those who do not know it Half-Life: Decay is the exclusive co-op game that has been added to the PlayStation 2 version. It's actually quite nice to play (The co-op is good the added DeathMatch is terrible). Here the news from VERC:

Originally Decay was an exclusive official add-on for PlayStation 2 version of Half-Life developed by Gearbox software. Our port is based on recreation of PS2 version of the game.
Decay will place you in the shoes of doctors Collette Green and Gina Cross, colleagues of Gordon Freeman, who are also working at the Black Mesa Research Facility.

Check the VERC for more.