Aridia is a ROM editor for Phantasy Star III. It has a mostly-friendly interface that makes editing incredibly easy. No messing with hex editors, decoding values, or any of that. If you can click and type then you can hack away to your heart's contentment. It currently supports editing: character stats & inventory, enemy stats, all game text, title logo & font, weapons & items and more.
There are currently two branches for Aridia:
-1.x: Built against .NET 1.1, targeted for Windows XP SP2 and 2000. This is frozen unless any major bugs are found.
-2.x: Built against .NET 4, targeted for Windows XP SP3 and higher. This is the active development branch, all new features will be against this version. This works with Mono but the UI looks a little funky.