From Diablo Wiki
ATMA is a Single Player muling program for Diablo II by Hakai_no_Tenshi. It stands for "A Tenshi Muling Application" and is designed to help you organize and categorize your Diablo II characters, mules and items. The idea behind this is to be able to read your character/item files quickly without having to open them up in the game and to be able to transfer items without the problems of previously available editors/muling programs.
The program, tutorials and other information related to this is available on Diabloii.net: