World 1: Prehistorik 2 (Super Mario Bros. secret world)
World 2: Kirby
World 3: Cheetahmen (Justin.tv secret world)
World 4: VVVVVV
World 5: Wario
World 6: Castlevania (Pokemon secret world)
World 7: Milon's Secret Castle and Metal Gear (World of Warcraft secret world)
World 8: Megaman
World 9: Kid Icarus
World 10: Ninja Gaiden
World 11: Mario Desert
World 12: Final Path
There are also secret worlds in boshy which have secret bosses
Secret World 1: Super Mario Bros. - This area is located at the end of world 1. You'll need the gear from World 7 to access this area. Completing this area will unlock Mario as a playable character. There are no bosses here.
Secret World 2: Justin.tv - This area is located in world 3, two screens after the entrance to the Mario boss battle. You'll need the cake from World 5 to access this area. Completing this area will unlock Kappa as a playable character, who is the boss for this world.
Secret World 3: Pokemon - This area is located under the water in world 6, before the screen where you enter the Vic Viper Starfighter from Gradius. On the left side of the screen is a water current, not moving and slightly lighter than the rest of the water. Under the water are 2 buildings taken from classic Pokemon games, one for entering and the other for exiting the world. There are no bosses here, and the only real purpose is to unlock Ash Ketchum (Red).
Secret World 4: World of Warcraft - This area is located in world 7, under the screen where you get in the plane. Completing this area will unlock a WOW orc as a playable character. The bosses in this area are the Cheetahmen and their enemies.