Lunar Magic is a level editor for Super Mario World (it works with both the US and Japanese versions of the game), and is pretty impressive. It lets you modify most anything in the game, including overworld maps, title screen, backgrounds, forgrounds, enemies, etc. It even lets you import new sprites into the game. People have been hacking ROMs for years but most of the time they're either done by hand or though some weird array of cryptic looking programs. Lunar Magic is much more user friendly in this regard.

Since it's PC only I tried it out through VPC on my Mac and it works pretty well. The interface takes a bit to get used to, although that's expected, but for what it does it's rather impressive. I did an initial test of changing and adding a few things to the first level, and it worked like a charm. It does seem to bloat the ROM file however. Three minor changes ended up doubling the file size. This is a small price to pay, if any.

I think I'll be playing a bit more with this in the future. I have a few good ideas of how to use this for my next ASMBLR project. Could be an interesting endeavour if I can get it all to work.