BattleBlock Theater

There is one secret level hidden in each world, hidden within one of the story levels. The location of access to a secret being nearby is indicated by a one-eyed flamingo thing. This will unlock immediately upon entering the secret level.
List of secret level locations: 

• In level 1-7 drop down below the first laser through the fake floor.
• Level 2-2 After the extended corridor of hot coal blocks, break the second explosive block over a hot coal block, then launch through the fake floor into the portal. Go all the way to the right to activate the secret level
• 3-9 Jump off the dart fired by the enemy onto the wall to the platform on the left. This after the long corridor with lasers on the floor and ceiling.
• 4-7 At the very beginning use the hot coal blocks jump up and grab the block. Go back to the start and wait until the laser fires, then immediately drop down onto conveyor belt 
• and then throw the block onto the conveyor belt to the left. Follow it and grab it again, then throw it so it lands on the conveyor belt at the very bottom. Follow it all the way to the end, then jump up to the platform to the right.
• 5-8 After going through the teleporter, use the last launcher at the end of the belts and disappearing blocks, to go through an invisible wall and reach a higher area.
• 6-2 After you get the helipack, boost across to the hot coals and launch off them, then use the heli pack to fly over the second set of coals and land on the one block with no spikes on the far right. After you drop down go to the tree on the left
• 7-6 After activating the circular saw that press the two switches, go to the start of the path that led to the jet pack. Jump over the first saw obstacle then jump up to the area that is now accessible behind the brown disappearing blocks.
• 8-3 after the long corridor with lasers where you avoid on the top and bottom by climbing on the rails, jump through the fake wall between the laser transmitters