The peerless TIGSource led me to this wondrous discovery: an open-source (and freeware, natch) XCOM variant of considerable ambition. UFO: Alien Invasion, for that is the name of this beast, has taken a rather more serious approach to the subject matter of alien invasion than one might expect, with a purported 'hard science' fiction and a whiff of the dreaded realism to authenticate the extra-terrestrial shootings. Needless to say, it still delivers the kind of turn-based agonising, base-building, and team-equipping fiddliness we've come to love. I've not had a chance to play more much - had a few other distractions today - but I expect I'll post something a little more detailed later this month. As I understand it there's a new version due soon and they'll need beta-testers.
The downloads page is here, and you'll need to select the operating system from the funny drop-down menus. It's 261mb big, which is the equivalent some amount of freeware games... I don't know. I'm so tired.