I thought I'd resurrect this old thread to say that I'm playing X-COM: UFO Defense again and loving every minute. This game came out in 1993... can you believe that? To this day I truly feel that no other game in this genre has surpassed it.
The game is considered abandonware now, and as such you can find it on several abandonware sites. There are a few caveats: if you have Linux or Windows Vista, you have to use the original DOS version + DOSBox, not the newer Windows version. This is because the game resolution is something like 320x240 and if you run it in wine on Linux, it will be the size of a postage stamp. You can't run the Windows version on Vista at all, it just doesn't work.
If you have Windows 95, 98, or ME the Windows version will work fine. If you have XP, you need the Windows version with the X-COM XP fix EXE file (freely available).
If anyone hasn't played this game before, I implore you to try it. I truly feel it is the best TBS game ever made. Hell, it's one of the best PC games ever made. The graphics are now rudimentary, but adequate. The gameplay is where it really shines; it is superb.
It's not an easy game, I'll warn you, and the interface can take some getting used to. But I can help anyone out with everything from interface help to strategy to mods to getting the DOS version working in DOSBox.