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 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.

Stumpy's War - a commander's log.

Daishiva's MapView, which lets you view, edit, and create terrain tiles, UFOs, etc.

DOSBox, a DOS emulator wrapper for XCOM in WinXP. Also see notes here.

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 = X-COM: General =
-[http://www.the-underdogs.org/game.php?id=4966 X-COM for Win95 from Underdogs]
 http://www.strategycore.co.uk - StategyCore (formally X-Com Tactical Command, but merged with a few other sites).