I'm surprised I hadn't heard of this before today - I've followed the Half-Life series quite closely, and I had no idea they had an officially released game on the PS2 that wasn't available on PC.
Anyway, this is being ported to the HL1 engine, for the pc, by a team of modders - and it looks like they're expecting to release soon. The game will feature multiplayer coop (since Decay was originally a coop game) and nine levels. They're releasing it in just a few weeks' time.
http://decay.half-lifecreations.com/
Half-Life: Decay is an add-on included in the PlayStation 2 port of the first-person shooter computer game Half-Life, developed by Gearbox Software and published in 2001.
Like Gearbox's other expansion packs Opposing Force and Blue Shift, Decay returns to the setting and timeline of the original story, but with different player characters: two female colleagues of Gordon Freeman, Dr. Gina Cross and Dr. Colette Green. Dr. Gina Cross is the model for the Hazard Course hologram, and can be seen at a point in Blue Shift. Dr. Green is a new character invented for Decay.
Although Gearbox completed a PC port of the game, Decay was never released "due to powers beyond Gearbox's control". As such most Half-Life players never got a chance to play through it. Decay is also not available through Valve's Steam service. Decay is, however, being ported by fans, at http://decay.half-lifecreations.com for the Half-Life engine in late 2007. If successful, they hope to remake it for the Source engine. The team has entered the beta testing stage and an official release date has been announced. The port will be released on friday, 16th November 2007.
- Wikipedia
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Heh! I played this years ago on a friend's PS2. Doesn't really add anything to the original game, nor improves on it significantly, so this mod would seem a tad meaningless.