Need help with that map you're making? Or with the alpha layer of a texture? Or managing the smoothing of a mesh? These are the tutorials and forums that can make map development go that much smoother.
*Tip #1* On the forums, before you ask your question, hit the 'search' button and run a search for your question. Odds are someone's already had the problem and gotten the answer.
*Tip #2* Don't ask about white viewports. See Tip #1.


AngelMapper - A well-respected mapper and coder in her own right, she has some quality tutorials that can help you out.
Architectronic - Although the pages are somewhat dated now, this is an excellent place to start if you're just a beginner.
BeyondUnreal - Their 'Mapping' forum in the Development section is also a good place to ask questions. Make sure to 'Search' first as your question may have already been answered.
Epic Games - Their 'Level Editing' forum in the UT2K3/UT2K4 section is a good place to ask questions.
Hourences' Tutorials - In the Team Phalanx section of his site are some great tutorials..
Myles Lambert's Tutorials - Some UT2004 odds and ends tutorials.
NaliCity - The 'Level Editing' section of the forums are a good place to ask for help.
UCMP - The guys here are nice enough to help out those who come searching for answers. - Links to other useful, more specific tutorials from various community contributers.
Unreal Developer's Network - The 'online manual' for the Unreal Editor. Lots of great info here. It's made by the people who made the game - of course it's helpful.
UnrealPlayground - One of the best forums to ask for Unreal Editor help, and also for help with content creation, ie: textures and meshes.
Unreal Wiki - It can take a little getting used to navigating through it, but the information is great.