http://tcrf.net/The_Cutting_Room_Floor

An excerpt

"Super Mario World was Nintendo's first title for the Super Nintendo, and it was rushed to release for this reason, leaving a lot of unfinished content.

It contains a wealth of unused levels, some of which are from extremely early in development; to unused objects, including a classic Piranha plant, to a ceiling full of Swoopers — all the way to an unused screen-sized cage!

This, combined with a lot of interesting prototype graphics from the SNES Test ROM and magazine preview screenshots, reveals that Super Mario World was originally intended to be a very different game — Super Mario Bros. 4..."