There are quite a few X-Com sites out there. They vary in how useful they are, how extensive they are and how good they look, but all have some use. There are also some very interesting X-Com related games in development. You should find links to them all here, but if you see one missing, please tell us. Besides that, thanks to Zombie for providing us with the links!

Games In Progress

There are many X-Com inspired games in progress. Some are commercial, while others are fan-made. We'll also keep you posted on their progress in the news. Please note that some of these have beta versions available for download!

X-Force: Fight For Destiny (playable)
This is a remake based on the two first X-Com games. If you haven't looked at it already, do so now! There's a beta version available for download at their site.
UFO Alien Invasion (demo)
This is a fan-made game inspired by X-Com and built on the Quake 2 engine. There's a demo version out, and it's quite nice! Head over to their page to read more and download the demo!
Invasion2021: Last Line of Defense (beta)
Invasion2021 is a remake of X-Com with some new features added. It is currently in a beta stage, and several features are yet to be implemented, but it's progressing nicely. The site is in Russian, but you can download a version of the game that supports English.
UFO2000 (playable)
From their site: "UFO2000 is free and open source remake of tactical part of 'X-COM: UFO Defense' game. It currently has no Geoscape and economics, but it has the feature many X-COM fans dreamed for - multiplayer support"
X-Com: Tactical (board game) (playable)
From their site: "X-Com: Tactical is a tabletop war/board game based on the popular X-Com: UFO Enemy Unknown (UFO Defence), computer game by Microprose. You play either the commander of X-Com, the Extraterrestrial Combat unit, or the alien commander come to conquer and enslave the Earth."
UFO: Extraterrestrials (none)
Development of this commercial turn-based strategy game began in 2002. It will feature excellent AI, destructible terrain and active NPCs like the police, army and civilians.
Project Xenocide (preview build)
This project is trying to recreate the spirit and feel of X-Com: UFO Defence (UFO:Enemy Unknown). It's fan-made, and anyone is welcome to contribute. Join today!
X-COM.Net Turn-based game (playable)
This is a turn-based game inspired by X-Com under development. It needs Microsoft's .NET Framework installed to run!

Essential Sites

There are many fan-sites out there, but some are more comprehensive or otherwise important than others. They are listed below.

XComUFO.com
This is probably the most extensive X-Com site out there. You can even play UFO:Enemy Unknown on site! UFO floorplans, strategy guides, downloads, news, forums and so on. They got it all!
Danial's UFOpaedia Website
This site has the entire UFO:Enemy Unknown UFOpaedia online. Absolutely all the information that you find in-game on crafts, base facilities and so on can be found there. Recommended!
Gareth Griffiths X-Com Wikipedia
This encyclopedia for X-Com aims to gather all relevant information on the game. It was started in February of 2005, and already has a lot of good information. Anyone can contribute, so get to work!
XcomUtil by Scott T. Jones
This is the official site for Scott T. Jones's XcomUtil. This neat utility enables you to do all sorts of crazy stuff with the X-Com games. This tool is essential for any X-Com fan!

Fan Sites

Unlike official sites, there are a lot of good fan-sites out there. It's been a long time since we last saw a proper X-Com game released, but the fans haven't forgotten.

X-COM Support

Arky's X-COM Page
This page has a lot of good information on X-Com. Strategy guides, downloads, timelines and more can be found there.
Devisraad X-COM Page
This site has some good downloads for the X-Com games, including mods.
Gomisan's X-Com Website
This site have some good X-Com information, and also cover other games Gomisan like. And in case you think there's something familiar with Gomisan, he also made the Silent Storm site S2HQ.com

The X-COM Games

General sites (cover more than one game)

XCommand
Another extensive X-Com site. Some of it's features like downloads are unavailable at the moment, but most of it is still accessible. It has screenshots, weapon tables, and base facility information, and the site covers all the X-Com games.
X-Com @ llamalicious.com
This site has a lot of good strategies for the X-Com games, as well as other good info.
NKF's Website
This site has some very cool X-Com avatars, as well as a few crazy challenges. Are you man/woman/alien enough?
Skyranger Central
Skyranger central is a fairly new site dedicated to the X-Com games. It contains forums, videos, gallery, reviews and news with more to come.
The XCOMainiac Site
Not much here except an excellent history of the X-COM series. A must read for any discriminating fan.
X-COM Russian site
Lots of great stuff here: UFO floor plans, a research tree, game information, strategies, tactics, screenshots and UFOpaedia. And although the site is in Russian, navigating the areas is fairly easy.
X-COM Fan Site
Even though this site is in Slovakian, it is one of the best sources of facts anywhere. It covers all the X-COM games in-depth as well as sections on the UFO series.

UFO: Enemy Unknown (X-COM: UFO Defence)

Online Ufopaedia
Unlike other "chop job" UFOpaedias, this is actually an unedited version culled directly off the game. Since it uses the same screens as in the game, navigating to the particular section is intuitive. All the normal information can be accessed through hotspot links in the menu.
EldritchEvil's Battle Report 1, 2, 3, 4 and 5
This site contain battle reports from various games. EldritchEvil has written reports from his UFO:EU missions. They are a combination of explaining the game and fiction. Well written, and great if you feel like a different sort of fiction!
X-COM: UFO Defense Website
Another very nice website! It has some info and files, a walkthrough, a research tree, and a UFOpaedia that rivals the best out there. In the upper left is a picture of Mars. Surrounding it are the four various icons representing the different sections. Click on those smaller icons for access.

Apocalypse

X-Com Apocalypse source (Link currently inactive).
If you're looking for information on X-Com Apocalypse, and you can't find it here, chances are you'll never find it.
Necrom's X-COM Apocalypse Training Center
Obviously, it's a strict Apoc website, but it is excellent nonetheless! It has separate pages for the Enemies, Tactical, Research, Equipment, as well as a Message Board and downloads.

Other

The Last Outpost
Cyke has done several very good interviews on X-Com. There's a series of 3 interviews with David Ellis that anyone with an interest in X-Com should read!
David Ellis' Website
Dave was lead designer on X-Com: Interceptor and the defunct Genesis. He also wrote the official X-Com strategy guides. If you wish to know more about him, his homepage is the best place to start.

X-COM Fan-Fiction

X-Com @ FanFiction.Net
This is where you'll find all the X-Com related fan-fiction available at FanFiction.net.
Some very good stories there, like "XCOM: The Unknown Menace" by Hobbes.
Scifi Emporium
This is Daniel Blakemore's Scifi Emporium, a site dedicated to fan-fiction. It's got a lot of good X-Com fiction, as well as Warhammer 40k and Battletech.
Combase: Nova
Another great fan-fiction site. While it is a little hard to navigate, there are plenty of stories to pique your interest.
La Pagnia de Fernando's site
Tons of great stories written by this talented individual can be read both online or downloaded.
Moloch's Place
One of the rare places which feature Apocalypse and Alliance fan-fiction.

X-COM Modding

X-COM: Area 51
This is Hobbes X-Com site. It has terrain, buildings and other add-ons for the X-Com games, as well as fiction and other goodies.
DaiShiva's Website
This site has some good X-Com editors, as well as information for modders.
Munkylord's X-com mod page
This page contain several mods that Munkylord put together for X-Com. Mostly weapons.

"Official" Sites

Since MicroProse is long gone and the Gollops do not own the license, we'll end up listing the "official" sites here.

Atari
They currently own the X-Com license. Chances are slim to none they'll do anything with it, and our ultimate goal is to change that. But for now it's dead in the water in Atari's cellars.
LaserSquadNemesis.com
The Gollop brothers (yes, they made X-Com) have spent the last few years making LSN. It's a good game. Who would believe e-mail games are fun? There were talk of a single-player version of LSN at one point but it's been awfully quiet lately...
X-Com Enforcer (Link currently inactive).
This is the official X-Com Enforcer site. It has UFOpaedia, downloads, screenshots, wallpapers and more. Don't know about the game, but the site is quite good!