The X-Wing community is a lot smaller than it used to be sadly. There are still some fan sites out there with passionate creators dedicated to enhancing X-Wing Alliance. Check out their sites.
X-Wing Alliance Upgrade
The current centre of the X-wing community. They continue working to replacing the current XWA Opts with new and vastly improved versions. Also it has an active Forum.
Vince T's X-Wing HQ
General Vince Trageton's site. A member of the XWAU and an excellent Opter! Take a look at his great work and his special massive Blue Phantom Project
DTM is a really great creator and has expanded the XWA universe in many ways. If you are a fan of The Clone Wars, Rebels and the prequels then check out his amazing opts.
TIE Fighter Total Conversion
This mod is a total conversion of the original 1994 game TIE Fighter into X-Wing Alliance. Offering all original 13 campaigns and expansions as well as a completely reimagined campaign of the first 8 campaigns alongside it.
X-Wing Game Series
Michal's Twitter page covers XW series screenshots, concept arts and collectibles. Loads of unseen pictures and information from the games. Well worth a look.
Jefe Pícaro'sssite features the X-Wing Historical Museum. He has files from many dead XvT/XWA websites. So if you are missing something X-wing then look here!
Troy Dangerfields website. Creator of AlliED and XvTED. If you want to create missions then you will need an Editor. This was the original and still one of the best for missions.
The official Star Wars website. All the news straight from the source. Details and images on all the movies, TV shows and collectables from all areas of the Star Wars universe.