Level Editors
Emulation

Lunar Magic

An excellent Super Mario World level editor.

Hacks

Mario Improvement

Mario Improvement is a SMB level editor for use with the SMB ROM with title screen.

Levels
You'll have to use some sort of ROM patching utility to patch these files to the ROM.

Mario Improvement³

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.