Hi everyone! I'm new at this, but I have to start someday, so here goes.
A while back, I was trying to find a level designer for New Super Mario Bros. for DS. I didn't find it, but I did find a sweet Mario and Luigi game and level editor.
It has a pretty good 1-player story mode, but the best part is the level editor. The fun part about it is the near-unlimited different levels you can make, but it's also cool because of the drag-and-drop level designing process. I've made two interesting levels, and dozens are being shared on the Internet. Please try it!