Sploggy

 TIPS:
 At the beginning of the first level, let yourself fall into the
 water - you'll be thrown upward. Now try to jump back into the
 black hole that normally takes you to level 1. If you succeed,
 you'll enter a bonus world. (Does not apply to the demo version!)

 In the 3rd level, let yourself fall to the bottom right at the
 level entrance and press the joystick slightly to the left while
 falling. You'll land directly at the level exit.