What are 3d shooters without there fanpages?.... I tell you... nothing!
  

Half Life

Half-Life Files News and a lot of maps and mods to choose.
HIT Forum Replacement Models, and a lot of WIP show-off.
Sven CO-OP My Favorit Mod for HL1 its Coperative gameplay!.
The Specialist Best deathmatch gametype styled mod ever for HL1.
  

Serious Sam

Seriously! Best SS page, with lot of addon downloads for SS1 and SS2.
DK's Planet He made some really wicked mods for SS1 and SS2.
  

Unreal Tournament

Byhond Unreal Very nice news page about Unreal universe.
UT Files Good Place for maps and mods of all kind.
ChaosUT2 One of the best UT2k4 mods out there with lots of new stuff.
U4ever It was the best mod for Unreal and UT. And its still a very nice mod for UT2k4.