http://www.strategycore.co.uk - StategyCore (formally X-Com Tactical Command, but merged with a few other sites).
http://www.xcomufo.com - this is currently the best source for all x-com games and also got a nifty forum.
Remakes and Inspired Games
- http://www.smksoftware.ru/pufo.shtml - Pocket UFO by SMK Software. Beta release, screenshots.
- http://ufo-extraterrestrials.com/ - by Chaos Concept. Unknown 3D engine. Screenshots, Czech forums
- http://ufo.myexp.de/ - TBS based on Quake2 engine. Pre-demo, screenshots
- http://ufo2000.sourceforge.net/ - extending original data files. Beta release
- http://www.ufo-aftermath.com/ - as published by Altar. Full Commercial Release
- http://www.projectxenocide.com/ - Alpha release
- http://www.xforce-online.de/ - German site, version 0.900 release
- http://www.invasion2021.narod.ru - Russian site, English translations, screenshots, pre-demo.
- http://www.xcom-lasthope.com - CounterStrike-based at the moment. Awaiting a good HalfLife SDK programmer to wean it off CS.
- http://dl.openhandhelds.org/cgi-bin/gp2x.cgi?0,0,0,0,20,2712 - Fully functional recompilation for GP2X handheld.
Stumpy's War - a commander's log.
Daishiva's MapView, which lets you view, edit, and create terrain tiles, UFOs, etc.
X-COM Trilogy UFOpedia - Game graphic UFOpedias for all 3 Xcom games
http://www.xcomufo.com/x1ufopaedia/index.html - with game graphics
http://www.ufopaedia.com - Danial's UFOpaedia