Unreal Developement Kit

[Mr Bean]

A free UT3 editor. IMO much better and than the one that comes with the game. The UDK versions are also updated very frequently (over once a month). Anyone interested in taking it for a whirl... heres the July 2010 beta version:
http://www.udk.com/download

[Variasaber]

UDK is basically the editing software for making things within Unreal Engine 3, but outside of UT3.

I have UDN, which is the same but for Unreal Engine 2 / UT2004. My comp can't handle UE3