If you want a more informative thread go to DaVince's post.
This is the first one I found, at the very top of the green/blue level select room.
Overworld
Found above the place you spawn the first time.
On top of the castle. If you stick to the purple scarecrow man you get to the haunted ps1 room and skip the monkey king.
That one spooked me.
You get to this room by sticking to the purple guy on top of the castle. Idk if this is a secret or just a placeholder for something.
Castle
This little guy is right behind you when you enter the castle.
When you enter the castle turn right and walk into the wall where the torch lever is.
Green Level
In the green level, this one spooked me.
Lever in the green level to open the passage to the red level.
Blue Level
In the blue level on top of one of the snowball dispensers.
Little peeb's inside of a snowman in blue level.
Behind the blue level entrance.
On top of the same building from the previous screenshot, covered in snow.
Idk what that is.
You can't miss this cat.
The red key for the red level. Like I said before, refer to DaVince's thread for more info.
Make sure you save before you beat the red level, because it sends you back to the most recent lamp post.
Checkerboard Place
Off to the side somewhere when you cross the non-solid fences.
If you go around the outside of the fences, you'll find this.
Drop down behind the fences and find the test level portal.
Go inside for some trivia (I think)
I saw this from DaVince's post, if you hold down the right mouse button you get teleported to a secret level, at the end you pick up a cube.
You can see a spooky version of the title screen if you exit to menu after picking up the cube.
I really enjoyed the game. Everyone should play it.