This is a comprehensive list of all the game's levels and their creators.

W0: Totally Tame Islet
0-1: How Do I Mario Bros.? by halibabica
0-2: Princess Etiquette by halibabica
0-3: Secret Agent Training by halibabica
0-4: The Power of the Stars by halibabica

W1: Green Grass Island
1-1: Hillside Streams by El_WaKa
1-2: Colorful Playground by Frogge
1-3A: Fungal Forest by Blobofgoo
1-3B: Tall Tops Forest by Illy
1-4: Autumn Lake by Davenport
1-5A: Mushroom Samba by Solitayre
1-5B: Mushroom Steppes by Seiromem
1-6: Burt the Unabashed's Fort by halibabica
1-X: Tempest Valley by pyrodoom

W2: Deathly Desert Island
2-1: Ghost Coast by kentona
2-2A: Forgotten Dunes by El_WaKa
2-2B: Tokotoko Desert by Frozen
2-3: Mirage Manor by halibabica
2-4A: Doom Dunes by outcry312
2-4B: Nighttime Oasis by UPRC
2-5: The Grand Pyramid by Blobofgoo
2-6: Sinister Crypt by Jalex
2-X: Twilight Desert by Davenport

W3: Giga Growth Island
3-1: Wetlands by Davenport
3-2: Jungle Rhumba by Fflo
3-3: Green Thumb by jackalotrun
3-4A: Big Blooper Basin by Ratty524
3-4B: Piranha Plant Pond by UPRC
3-5A: Overgrown Grove by Frogge
3-5B: Giga Funklen by Seiromem
3-6: Naval Piranha's Bog by Animebryan
3-X: Big Fat Phanto Koopas by halibabica

W4: Molten Mine Island
4-1: Mushroom Underground by Solitayre
4-2: Searing Skull Stepping by Ratty524
4-3A: Snake Pit by Punkitt
4-3B: Vanilla Castle by Deckiller
4-4A: Crazy Mine Madness by lonestarluigi
4-4B: Dynamite Grotto by Illy
4-5: Barrel Volcano by Animebryan
4-6: Blargg's Lava Lair by halibabica
4-X: Bonestrewn Barrow by Seiromem

W5: Slip Drip Island
5-1: Chilly Tunnel by Fflo
5-2A: Frostbite Valley by Jparker1984
5-2B: Slip 'n Snow by outcry312
5-3: Colonel Cryo's Cavalry by Ratty524
5-4: Carps Against Humanity by NeverSilent
5-5A: Frosted Massif by Frozen
5-5B: Ice Climbers by Davenport
5-6: Col. Cryo's Cruelly Cold Cathedral by halibabica
5-X: Super Penguin Bros. by Solitayre

W6: Sky High Island
6-1: Lakitu Rampage by Jalex
6-2: Snow Mountain by Davenport
6-3: Scurvy Skyrates by halibabica
6-4A: Nifty Nimbus Ride by Illy
6-4B: Donut Fall by kentona
6-5A: Bill Bouncing in Billowy Brumes by pyrodoom
6-5B: Turbulent Temple by Seiromem
6-6: General Fly Guy's Air Force by Animebryan
6-X: Rockin' Roll Ridge by Ratty524

W7: Pyre Fire Island
7-1: Blazing Hot Inferno by Davenport
7-2A: Fiery Flumes by Seiromem
7-2B: Dragon Dungeon by NeverSilent
7-3A: Fire Temple Redux by Deckiller
7-3B: Boom-Boom Bastille by Ratty524
7-4: Podoboo Peak by Illy
7-5: Picante Pass by Jalex
7-6: Devastation by halibabica
7-X: Volcano Lotus Valley by UPRC

SPOILERS!