Abuse/Secrets

Welcome to warp zone
On the first level (tutorial level), right after the first save station and before you pick up the speed booster chip, you will notice a damaged ceiling. Blow it up and use the jump booster to enter the secret area where you will find a door leading to three exit portals. These portals, from left to right, will take you to level 2 (skipping the rest of the tutorial), level 3 or level 4.

"Secret level"
Not really a full level, but a unique hidden area. When you reach the exit of level 14, blow up the wall to the left of the exit portal. You will find the entrance to a red, alien area unlike anything else in the game.