Map Editing
BlackSway, UED Resource Lab, Unreal Developers Network, Unreal Technology presented by 3DBuzz.com, Unrealized, Urban Distortion, UT City, UT Editing, UT Music, Wolf's Unreal
Lode's Unreal Zone
Includes editing tutorials for lighting and movers.
Oddities
Coding tutorials for creating unique game modifications.
Sinnical's Unreal
Information on maps, screenshots of works in progress, links, and news.
Squirrelinabox Technologies
Offers maps for capture-the-flag as well as a report for Unreal Tournament 2003.
The Mutation Device
Tutorials, utilities, and forums devoted to creating mutators.
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.