The diablo II editors are alright. they're all third party, but im not sure if you are only concerned with editors that come with the game. once you download all the different tols, it's actually quite easy to change things. the data (items, abilities, item drops etc.) is kept in tabulated text format, and they have a special spreadsheet like thing for that. they have a special text editor, for editing the text in the game, like the names of the items, speech, locations etc. of course they also have converters, for the animations and what not. changing the animation "control" files (i forget the extension) is rather tricky though. you need a hex editor, and a tutorial. you just can't work it out for yourself. then there's the character editor, for once you have started a char and want to change, say, how much gold you have, what skills you have learned, what items you have on you, what quests you have done.
of course it's been years since i even touched the game, so they may have changed quite a few things. for more info:
www.d2mods.com not sure if it's still alive though.