The Unreal Admin Page
BeyondUnreal, BeyondUnreal wiki

Unreal Redux is a fan made project to remake the game Unreal which was released in the year 1998. This remake will stay close to the original game but will be enhanced with new features. This remake is based and runs on Unreal Engine 1 which is extremely enhanced by the new oldunreal’s 227 patch. Therefore, the remake is able to catch the flair and atmosphere of the original Unreal while beeing improved with new visual effects and gameplay mechanics.

oldUnreal for Unreal patches after the last official patches.

SkaarjMonuments.com is a site devoted to the creation, sharing and discussion of Unreal 1 maps.
UnrealSP.Org “The premier Unreal single player site”

UT files
Loki Games - Unreal Tournament for Linux FAQ
Liandri Archives - Unreal Tournament
UT99.org - Unreal Tournament Community
Unreal Central Unreal Central - Downloads

R3uk.com - Setting up a dedicated Linux Unreal Tournament server
TLDP - Game Server HOWTO - Unreal Tournament
Bigsmoke - Creating an Unreal Tournament 99 server in Linux
The Unreal Admin Wiki - Server Install linux
Open Security Research - Unreal Tournament 99 Server On Ubuntu 12.04 (AWS)
UT99.org - Unreal Tournament Community - TUTORIAL: Unreal Tournament Audio & Visuals Tweaking Guide
Steam - Solution: How To Solve UT Running Too Fast, and get Widescreen!!
Ubuntu Linux - Community Wiki - Games/Native/UnrealTournament
GitHub - soifou/install-ut-goty-debian-jessie-64bits.md

UTPG.org provides version 4.40 and 4.51 patches.
An enhanced OpenGL renderer for Unreal Tournament
kentie.net - Unreal engine Direct3D 10 renderer

The OpenUT Project was formed to develop libraries, executables, and tools relevant to the Unreal Tournament engine. The project focuses on improving the quality of the UT Linux port through open source development, and aims to create a community to help support and develop UT for the platform.

GitHub - alexstrout/foxWSFix-UT99 Improved widescreen support for Unreal Tournament 1999.

Unreal Textures voor Unreal en Unreal Tournament.

Sinistral's Unreal - Maps Unreal Tournament maps.
J's Maps - Custom 3D models, level designs and programming for Unreal and UT by J. Martin
UT99 Maps - A personal collection of UT99 maps

Tactical Operations 2.2

Found by searching “unreal tournament” on GitHub.

carlio/ut-server-setup scripts and info for getting Unreal Tournament GOTY or 2004 servers running.
alanjjenkins/archlinux-package-ut2004-steam An AUR package for installing the native Linux version of Unreal Tournament from the Windows version in Steam using steamcmd.
FaultyRAM/Ut99PubSrc The Unreal Tournament v432 public source distribution, fixed for modern compilers.
stephank/surreal Reviving the original Unreal Tournament on modern systems.
GreatEmerald/UT3Weapons Unreal Tournament 3 weapons backported to Unreal Tournament 2004; cloned from Gitorious
infertux/ut2004-linux FAQ for running Unreal Tournament 2004 under Linux
moceap/ut Unreal Tournament Runner On Fedora
liflg/unreal.tournament_451-english_x86

For running ut99 with Docker.

adm1nx/docker-ut99 Unreal Tournament 99 server as a Docker container.
mgrenier/ut-server Dockerized Unreal Tournament (1999) server
zedr/docker-unreal-tournament-99 A Docker image for running Unreal Tournament 1999 in Wine.