It's almost unbelievable that modder Kaze Emanuar has spent 4,000 hours writing 100,000 lines of Assembly code to bring his vision of the perfect Mario game to life. Romhacks of this nature often reconfigure already existing levels to add more challenge, but Super Mario 64: Last Impact is so much more—it's an entirely new game, complete with its own story, inside of Super Mario 64.
If you watch the trailer above, you'll see little that reminds you of the original Mario 64. Instead, Last Impact is herculean effort to design an entirely new game with its own 130 stars to collect, 25 new levels, bosses, and power ups. There's Mario Sunshine's F.L.U.D.D. water cannon, but Kaze also added his own original ideas too. The Cloud Flower let's Mario make cloud platforms to walk on to help traverse big gaps, while the Bee Mushroom gives him wings.
The level design looks equally impressive, with areas themed after circuit boards, casinos, and Rainbow Road. Hell, you even go to the moon.
If you want to learn more about what Super Mario 64: Last Impact is all about, check out the wiki page which breaks down more of its features.