Lode's Unreal Zone
Includes editing tutorials for lighting and movers.

Mo's Maps
Offers maps, and map-making tutorials for Unreal and Unreal Tournament 2004.

Oddities
Coding tutorials for creating unique game modifications.

Sinnical's Unreal
Information on maps, screenshots of works in progress, links, and news.

The Mutation Device
Tutorials, utilities, and forums devoted to creating mutators.

Unreal Realm Of Concepts
Maps, tutorials, and ideas for level design.

Unreal University
Two-day tutorial covering techniques for creating mods for the Unreal Tournament 2004 game engine held at North Carolina State University in Raleigh, North Carolina.

UnrealWiki
Open community documentation for UnrealEngine based modification development.

World of Unreal Maps
Links, music, tutorials, and map and texture downloads.

Yahoo! Groups: Unreal Editing
Mailing list archive and subscription information.