Brutal Mario

It is a Super Mario World ROM hack created by a mysterious Japanese ROM-hacker who goes by the name of Carol. It's well known for being a hack with a large amount of custom bosses and ASM gimmicks, with its development being started when such things were but a dream in the SMW hacking scene. The hack contains several references to many other video games, from Final Fantasy, to Twinbee, to Kirby, to Mega Man, to even Cho Aniki of all things. Each level contains multiple enemies and even bosses. A level may contain more than 1 boss, though this is common.