The jury is, as far as I can tell, still somewhat out on the Games For Windows do-ever, but Microsoft's Steam rival has certainly been offering a few tempting mega-discounts lately. This time (as LewieP tips us off to) it's the original STALKER (Shadow of Chernobyl) for less than a pound/dollar. Only lasts for today, so get a wriggle on if you want it and aren't one of those chaps claiming a moral objection to GFW.
Shadow of Chernobyl, incidentally, remains my favourite of the three STALKERs, although I suspect I'd direct anyone new to the series to start with Call of Pripyat. Chernobyl's certainly rougher, but that's part of its singular charm for me. It's also a milestone game: the right shooter at the right time, a vital statement of what gun games could become, just before the vast bulk of them peeled off into either mega-budget console linearity or high-barrier-to-entry multiplayer fare. Shadow of Chernobyl remains a prophecy of what might have been. Perhaps even of what might yet be.
It's as essential a play as any game of the 21st century possibly could be, as far as I'm concerned.
(Oh, and grab the Stalker Complete mod anthology for it, won't you?)