I checked the links for this page on March 25, 2007, but I don't update it with new information or links anymore, since I no longer have time to play this game. But this is a pretty good list of stuff I found for UT2004.
UT2004 game stats - # of players online, gamemodes and mods played
UT2004 Tweak Guide - Tweak your configurations
boomtown.net ut 2004 tweakguide
Community Bonus Pack - extra maps, skins & more
Agent INI - tool to edit ut2004 INI files
mapmixer - edit map setups
Recommended Mods
Invasion Everywhere - mod that allows you to add monsters to any gamemode (ctf, onslaught, ...)
satore monster pack - adds the rest of the monsters from the original Unreal game
Fraghouse Mod Team - several mods for UT2004
UT 2004: UT Comp mod - another great mod
PlanetJailbreak - Jailbreak mod
Alien Swarm - topdown perspective shooter with coop gameplay
Chaotic Dreams - ChaosUT 2004 mod, tons of new weapons, including melee weapons
The Official UT2004RPG Website - RGP mod for UT2004
unrealdev.net - has the UT2003 relics mod which works with UT2004 also
Battle of the Sexes - class based mod, males vs females
LawDogs Mod - wild west mod
Unreal Fortress 2003/4 - team fortress mod
FREE MONKEY INTERACTIVE 2004 xmp
CTF 4 - play DM or CTF with 4 teams
Excessive - fast action for UT2004
More Mods
TrekUT - Star Trek mod (moddb link)
unrealxrt - motorcycle fighting theme mod
MonarchMain - Advanced Armor mod (new vehicles)
Red Orchestra - WWII mod on the Eastern Front
Atlantis - dead mod
Unreal Ops
ut2004troopers : Dawn of Destiny - a Star Wars mod
DeathBall - Drive the ball into the goal with your vehicle
UnWheel - racing mod
botmanager
The Wanderlust Project
bluemoonrising
unreal pexgames
Zionite War: Manifest Destiny
Robostorm, A UT2004 Mod
Archasis:Battle for Durian - Facts
Frag.Ops Official Homepage -- A total ut2004 conversion mod
SpeedTree - Download Executables
e2games
phase 3 finalists: non fps and mutators
phase 3 finalists: fps and gametype
phase 3 finalists: vehicle and karma physics
Bind the AVRIL and Toggle ThirdPersonView
H=getweapon Onslaught.ONSAVRiL
Z=Behindview 1 | Onrelease BehindView 0