If it's about Mulder or DD, they'll probably have it.
Last time I looked this site wasn't there. I don't know if the site was closed down(temporarily or otherwise) or if it's just my browser acting up. I'll try and find out.
If what you're looking for is information on the show's creator Chris Carter.
Duchovny'd Incorporated
Another DD devoted site. It looks really good, it's just too bad it takes quite a bit to load.
The Gillian Anderson Official Web Site. A rather extensive archive of sounds, images, interviews and whatever you'd like to know about GA.
Vince Gilligan is one of my favourite writers. In this site you can find biographical information and extensive information on all the episodes he wrote for The X-Files, or join the society.