X-COM: General

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 Altair. 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.

Stumpy's War - a commander's log.

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 == Fanfiction ==
+[http://scifiemporium.xcomufo.com/ Daniel Blakemore's Sci-Fi Emporium]
+[http://www.fanfiction.net/l/424/3/0/1/1/0/0/0/1/ Fanfiction.net section on X-COM]
 [http://www.geocities.com/TimesSquare/Lair/1798/xcomlite.html Fernando's Saga]