"SMB feat. Piranha Plant" is as implied by the title, a hack that replaces the Mario Bros. with playable Piranha Plants. The hack comes in two different versions in the form of patches labelled "plant_a" and "plant_b".
"plant_a" has Piranha Plant playable and fixes a few minor sprite/palette errors seen in the original game, but nothing more than that.
"plant_b" has the same content as "plant_a", but now with further modifications; some common world elements such as blocks, items and even non-playable characters have been altered, now referencing the titular Piranha Plant.
Please note that all screenshots shown here are of the "plant_b" patch.
No-Intro Name: Super Mario Bros. (World)
(No-Intro version 20130731-235630)
File SHA-1: EA343F4E445A9050D4B4FBAC2C77D0693B1D0922
ROM SHA-1: FACEE9C577A5262DBE33AC4930BB0B58C8C037F7