The extremely
versatile map editing tool included with
Unreal Tournament, and presumably with
Unreal.
Unlike most map creation tools, UnrealEd makes use of Constructive Solid Geometry. In its first iteration, the program was extremely buggy, and prone to crash. The second version of the program has reduced the number of crashes, making it far more user friendly.
Thanks to a strong fanbase, tutorials exist for almost any effect or situation one might wish to create within the game.