Here is a ROM hack for Super Mario Bros. that changes most of the graphics, all the level layouts, and some of the mechanics of play by altering a few of the enemies behavior.
Super Mario Bros. (JU) (PRG1).nes - GOODNES 3.14
CRC32: 2E1B6B3C
MD5: 94EDE9347C1416105F1C08EC26B5B73A
SHA-1: B1952F6EE88775CE4BAEC034A04E145AFB4930DF