[Download Link] - Also know there's a fairly detailed guide there too.
The first demo is only capable of editing items & enemies, but also features Sprite & Text viewers. It requires an English ROM of Golden Sun: The Lost Age, don't ask where to get one I can't tell you anyway.
Screen ShotsClick on the thumbnails to view the full-size image.
From left to right, top to bottom:
1) The Item editor, looking at the options it appears that I've turned corn into a lethal weapon.
2) The Enemy editor, and Briggs.
3) The Enemy Group editor. No idea why Saturos & Menardi are in The Lost Age, but it gives me an opportunity to make an epic boss fight.
4) The Sprite viewer, seen are a couple of the more unusual characters I found in The Lost Age.
5) The Text viewer, and what I can only assume is a message left behind by the game's creators/translators.