There is an open source version called UFO:alien invasion.
It's not complete but its allot of fun
http://ufoai.sourceforge.net/
My weekend disappeared. What wasn't already pre-booked to do other things was UFOAIed.
It helped that I almost immediately found a 'money bug' (I assume they know about it) that (it looks like, from experimentation) lets you start with as much money as you can sit there clicking to obtain and that it positively encourages Save Scumming on missions, so as to attain success (rather than abject failure), low/zero casualties (rather than winning but losing most of the team) or a near perfect assault (for as long as you want to gamble at doing it better). Or (for some cases) to pick and choose a better map/initial alien placement from the available set for that locale. (Sometimes I've deliberately sent a 'team' in consisting of a one-person 'super-specialist' on an interceptor, at the expense of diversity and multiplicity of units, because I have one of the appropriate type sitting in a craft within range configured exactly as I would wish.
Could do with a bit more help with managing resources, and (like original XCOM? ..I forget) having to reapply equipment to soldiers you took off duty (for medical/unit mix reasons) when you put them in a new craft is a bit tedious. In fact I've also taken to renaming all my hired soldiers with the likes of " (HW54)" after their name to indicate their main speciality and level, to aid with the shuffling and " [inj]" for those I want to stay in hospital (to save me time when scrolling the list, even though it displays health at the point of highlighting them). Also sending squadrons of interceptors to flying UFOs should be less tedious than clicking the UFO so many times and selecting the next appropriate interceptor down, or at least happen with time stopped so that you don't end up with a chain of aircraft (after all, they each have their own hanger/VTOL launch bay).
And I got an interesting error[1] twice, when saving. That was the first time I had a downed UFO location appear on the map, again and shortly afterwards when I had reshuffled the appropriate landing party and wanted to save progress to avoid the pain of doing that again. It seemed to respond best to [Ignore]ing, and hasn't seemed to cause any problems since. Certainly hasn't reoccured for any other UFO location (downed or discovered).
On the whole, a decent game, and a decent conversion, unless my memory fails me. It even seems to have the same issue/bug with setting off on a mission with a piece of equipment your soldiers can't
actually handle assigned to one of them as main weapon, and when you embark he's 'dropped' the item and you have to revert to the secondary weaponry or juggle auxilliary equipment between team members before engaging the enemy.
Now I must see if I've still got my original disks of the original game, somewhere, and if they work. Ditto for my First Encounters CD. Oolite having managed to absorb a significant proportion of another recent weekend, but leaving me yearning for the ability to land on planets/conduct inter-planetary commerce/nicer missions and fly and rotate through space with proper Newtonian motion...
[1] "Assertion Failed!
Program: C:\Program Files\UFOAI-2.2\ufo.exe
File: C:\Dokumente und Einstellungem\m...\cl_bas...ement.c
Line: 3414
Expression: *aircraft->aliencargo[l].alientype
For more information [blah blah blah, then Abort/Retry/Ignore buttons]"
Which, along with
minor linguistic anomolies (along with the repetative nature of some of the UFOpedia entries/reports) indicates the non-English heritage of the development team. But only spotted two actual errors, that I can recall. And one of those ("off" instead of "of") is common among the sloppier/mis-educated native English-speakers, so can't argue with that.