UFO: Enemy Unknown was released by Microprose over a decade ago. For those not familiar with it and it's predecessors, or for nostalgia, here's a thorough review of one of the all time great games. It is abandonware these days and as such can be downloaded for free although you'll need DOS emulation at the least to be able to play it.
The UFO series of games has a massive fan base and has such there is a number of fan-made games.
Probably the highest profile project is UFO:AI , a 3D remake of the original UFO: Enemy Unknown. Now it's come out of the shadows of closed development, the project is thriving and it is evolving into a showcase open source game. There's still a little way to go - you're going to struggle if you are not familiar with standard UFO gameplay - and there are frustrated players but their issues are being addressed and, with great graphics and great music, once playability is polished the final 2.0 release should be one to savour.
The road to the 2.0 release has been a fairly long one, and the developers relatively quietly released version 2.0-RC4 at the end of last month. (I don't remember seeing the announcement anywhere.) Call me old fashioned but I really think they need to just say, "Hey this is version 2.0 and the next release will be 2.01 or 2.0.1 or something." RC stands for Release Candidate, not for Remarkable Changelog. Anyway, I am digressing.
There is an even more ambitious open source UFO project - Project Xenocide. Again a 3D variant of the original game, this is a long term project with a steady history of planning and, lately, steady development. They recently added SVN tracking to their forum so activity is more obvious. Currently the only aspect of the game I'm aware that works is a gorgeous GoogleMaps-esque 3D Geoscape.
Some of the renders in the image gallery for Xenocide are of a very high calibre. I'm looking forward to seeing such high quality art incorporated in an open source game and I expect this game to emerge later next year in spectacular fashion.
The final part of the open source UFO puzzle is UFO2000. This is an old school project, taking the original game and it's graphics and turning it into a multiplayer deathmatch-style tactical UFO-fest. These days there is enough art that you do not need the original game to play UFO2000. However, whilst in its day UFO looked good, it looks very dated now. Despite supporting the use of scale2x to enhance the graphics a little, UFO2000 really shows the age of the original and will probably only appeal to hardcore fans - or those with older PCs like mine.
Like all great jigsaws, we complete it to find a leftover piece, X-Force. That's because the last open source UFO offering is Windows only. X-Force is a remake of the original and it's sequel, UFO: Terror From The Deep, modernising them whilst staying true to the 2D isometric tactical gameplay.
The website is mostly in German making it hard to really learn that much about it. From the screenshots, the game looks to be at a similar stage to Project Xenocide - more of a tech demo than a playable game. There is a download though so any Windows users or WINE experimenters might want to give it a go and leave a comment.
I'll be updating this article with screenshots later - I'm too busy falling asleep right now.