X-COM: General

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://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 0.835 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.