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 watter are 2 buildings taken from classic Pokemon games, one for entering and the other for exiting the world. There are no bosses here.
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.