A program designed to edit Super Mario World roms. It is versatile and user-friendly. Currently it supports level editing (both objects and sprites), importing and exporting custom graphics, overworld editing, palette editing, modifying pipe and door entrances and exits, message box editing, boss text editing, and almost everything else you can think of except importing custom music.
It was created by a programmer named FuSoYa.
This is by far the best level editing tool out there. Other programs pale in comparison. Anything you want to do to a SMW rom is within your grasp. The creator does however note that the program is not affiliated in any way with Nintendo. The warning should be well heeded as game hacking is of questionable legality at best.