Hooray for olden games! The entire X-COM collection is now on Valve's digidownloadwebtertainment store. Terror From The Deep's been there for yonks, but we finally get the first game, Enemy Unknown/UFO Defence. Which is possibly my most beloved game of all time, and I proudly take personal responsibility for getting it into the top 10 of PC Gamer UK's Top 100 list for the last two years running. If you're a real retro fetishist, you might also want lesser third game Apocalypse (which I've never actually played. Now's the chance, I guess). If you're a crazy person, you'll want rubbish FPS Enforcer and slightly less rubbish action flight sim Interceptor. It's $5 per game or $15 for the lot.
If you run Vista, you won't want any of them (I think - am double-checking). No doubt ways and means to get 'em running do/will exist, but right now it seems there's no official support. [Sad face].
Update - got word back from Valve - all of 'em except Interceptor should run fine on Vista, apparently. Woo! I know what I'll be spending the weekend doing, then.