Valve ERC Collective
Fantastic Half-Life editing site with very helpful forums. Recommended for newbies and advanced users alike.

MasteringUnreal.com
Mastering Unreal has a whole range of video tutorials and classes to help with learning Unreal Editor.

KungFuSquirrel
Home of the almighty Andrew Weldon. Perhaps one of the best level designers I have ever seen.

Johnsto.co.uk
Home to DaveJ, creator of Dust and Dust2. Has some very interesting articles on how he made these maps, as well as some thoughts on gameplay and level flow.