2015/Through the Warp Zone: Hacking Super Mario Brothers
The full video of this talk will be posted soon, so this will be more supplemental information than full exposition.
This talk was based on a blog post called "Through the Warp Zone" by Emily St. It explains a little bit about the minus world glitch, and how hearing about the glitch lead Emily to learn how to hack the game to access this glitch, and generate many more.
This video playlist demonstrates some of the hacks possible with this technique.
If you're interested in using similar techniques, you can download a hex editor like Hex Fiend (OS X only), as well as an emulator like Mednafen (with Homebrew, you can run `brew install homebrew/games/mednafen`).