Lunar Magic (SMW) | Mario Improvement (SMB) | Mario Improvement³ (SMB3) | The SMB2 Level Discombobulator (SMB2)
An excellent Super Mario World level editor.
Mario Improvement is a SMB level editor for use with the SMB ROM with title screen.
Mario Improvement³ is a level editor for SMB3. It features map screen support, and support for all levels.
The MI³ site also has a nice level archive.
The SMB2 Level Discombobulator
A SMB2 level editor.