Unreal Admin - the most important and useful site for setting up a UT2004 server.
UT2004.com - for downloading the dedicated server.
ProAsm Game Utilities - where I downloaded UT2Vote46. This site also has excellent tips and tricks for its installation.
Gameservers Redirection Page - what I use for my redirect server. For custom maps, I read reviews and note the good maps. Then, I download then from this site. It's significantly faster than almost any other server, and you don't have to screw around with file site memberships or subscriptions. I have yet to find a map, mutator, skin, texture, sound or music file that is not already on this server. Thanks to gameservers!!
Google - the BEST search engine out there.