GBAtemp member Schmendrick has released Level Editor, a new user-friendly tiled map level editor that allows you to build your own courses right from your DS. See below for a list of current features, and be sure to join the on-going discussion to get all the details about this homebrew project.
QUOTE: Current Features 11/06/11•Easy-to-use level editor with stylus controls
•Simple but efficient UI
•Generic level data format to be used in any game engine
•Show or hide grid, tile IDs or complete layers
•Level saving/loading to/from sd
•Up to 5 layers (last layer reserved for collision map data)
•Dynamic layer resizing (when possible, resizing will keep your level data in place)