An unlockable stage is a stage that is not available at the beginning of the game, unlike a starter stage, but must be received through other means.

Super Smash Bros.

Stage How to Unlock
Mushroom Kingdom Beat 1P Game with all 8 starter characters and play each starter stage in VS Mode at least once.

Super Smash Bros. Melee

Stage How to Unlock
Planet Zebes: Brinstar Depths Play 50 VS. matches
Eagleland: Fourside Play 100 VS. matches
F-Zero Grand Prix: Big Blue Play 150 VS. matches.
Kanto Skies: Poké Floats Play 200 VS. matches.
Mushroom: Kingdom II Obtain either the Birdo or Pidget trophy.
Past Stages: Dream Land Complete Target Test successfully with all 25 characters.
Past Stages: Congo Jungle Complete 15-Minute Melee.
Past Stages: Yoshi's Island Hit more than 1312.3/400m in Home Run Contest.
Special Stages: Battlefield Complete All-Star Mode using any character.
Special Stages: Final Destination Clear Event 51: The Showdown.
Superflat World: Flat Zone Complete Classic Mode with Mr. Game & Watch.

Super Smash Bros. Brawl

Stage How to Unlock
75 m Play as Donkey Kong 20 times on VS. Mode
Big Blue (Melee) Play as Captain Falcon 10 times on VS. Mode
Hanenbow Complete Event 28: Flower Blooms in the Echoes
Flat Zone 2 Unlock Mr. Game & Watch
Pirate Ship Unlock Toon Link
Green Greens (Melee) Play 20 VS. Matches using Kirby
Green Hill Zone Unlock Sonic
Jungle Japes (Melee) Play 10 VS. matches on Melee stages
Luigi's Mansion Play with Luigi in 3 VS. matches
Mario Bros. Complete Event 19: Wario Bros.
Pokémon Stadium (Melee) Play on Pokémon Stadium 2 in 10 VS. matches
Spear Pillar Complete Event 25: The Aura Is With Me

Super Smash Bros. for Nintendo 3DS

Stage How to Unlock
Balloon Fight Use Villager 3 times in VS mode
Dream Land Use Ultra Sword (Kirby's Final Smash)
Flat Zone 2 (Brawl) Unlock Mr. Game & Watch, then complete Challenges #1-35
Magicant Unlock Ness
Mute City Win 3 VS matches using Captain Falcon
Pac-Maze Use Super Pac-Man (Pac-Man's Final Smash)
WarioWare, Inc. (Brawl) Unlock Wario

Super Smash Bros. for Wii U

Stage How to Unlock
Congo Jungle (64) ???
Smashville (Brawl) ???
Pac-Land ???
Pokémon Stadium 2 (Brawl) ???
Duck Hunt stage ???

See also