Multiplayer is something many a X-COM fan wished for when the game was first released. The UFO2000 team took it upon them to provide what the developers didn't, and everyone is encouraged to check it out! You can find the official site here.
Project Xenocide still going strong:
Near the end of 2002 a group of dedicated fans started a project to recreate the feel and spirit of X-COM. It's been a long bumpy road, but the project is still going strong. You can find the official site here.
Pocket UFO for Pocket PC and PC:
Did you know that you can play X-COM on your Pocket PC? No? Time to explore then! Pocket UFO is a remake of X-COM: UFO Defense (UFO: Enemy Unknown) that you can download for free. And in case you're not the lucky owner of a Pocket PC, you can play it on the PC as well! Read more about it at the official site.
Even more X-COM related projects?
Sure thing! There are several other excellent projects in development, and you can find a nice list made by Zombie at StrategyCore.co.uk