Back-to-Back Mario Levels
I just downloaded a Super Mario Brothers ROM for my computer's NES emulator and I am loving it!!! I also downloaded a level editor along with it and that just makes things ten times as good. I'll put download links later when I get a chance.
However, right now I'm going to show you a website with all of the original levels mapped out on it. It'll help you beat that level you just can't beat. :-)