Editroid is a Metroid level editor that allows you to edit map and room layout, palettes, item locations, and much more. Editroid can also expand a ROM to accommodate larger and more detailed maps and even animated background tiles.

For more advanced users, Editroid includes a built-in assembler for both general purpose ASM hacks as well as screen-load routines that can modify the behavior of rooms in ways that simple data editing can’t. Version 3.7 allows you to add multiple ASM files to better organize more complex projects.

For more information about Editroid, see http://snarfblam.com/words/?tag=editroid.