The levels are divided into three pages in Brawl: new stages, Classic and Created. Head here more info on these levels. We have no idea if this is the final list or not - we’ll see once we get the game.

Images courtesy of Joystiq

Already announced:
Battlefield
Delfino Plaza (Mario Sunshine)
Yoshi’s Island 1
Lylat Cruise (StarFox)
Bridge of Eldin (Twilight Princess)
Smashville (Animal Crossing)
Rumble Falls (Donkey Kong: Jungle Beat)
Skyworld (Kid Icarus)
Castle Siege (Fire Emblem)
WarioWare
Pokemon Stadium 2
Battleship Halberd (Kirby)
Shadow Moses Island (Metal Gear Solid)
New Pork City (Mother 3)
PictoChat
The Summit (Ice Climbers)
Norfair (Metroid)
Mario Circuit (Mario Kart)
Frigate Orpheon (Metroid Prime)
Yoshi’s Island 2
Distant Planet (Pikmin)
Mushroomy Kingdom (Super Mario Bros.)
Corneria (StarFox)
Onett (Earthbound)
Brinstar (Metroid)
Rainbow Ride
Hyrule Temple (Legend of Zelda)

Recently announced/confirmed:
Green Hill Zone (Sonic)
Tetra’s Ship (Wind Waker/Phantom Hourglass)
Luigi’s Mansion
Electroplankton
Classic Mario Bros.
Final Destination
Port Town (F-Zero)
Big Blue (F-Zero)
Spear Temple
Pokemon Stadium 1
Jungle Garden (Donkey Kong)
Game and Watch