New Super Mario Bros Level Editor
Have you completed New Super Mario Bros for Nintendo DS? Looking for some new levels? Well, here's a nifty utility that will allow you to edit the existing levels in New SMB and play them! The PC app is called New SMB Editor, written by Treeki. After you edit your levels, you can either play it on your PC using a DS emulator or on your Nintendo DS using a flash card like the R4DS.
Here's the author's description of the app:
A level editor for New Super Mario Bros. It is still a work-in-progress, so many features are missing - but it’s still perfectly usable to edit level layouts! Visual Basic 6 source code is included. :) (For more information and a FAQ, go to http://tinyurl.com/yrxa9n)
Download the app here.