There are 10 hidden eggs (small light dots which crack when you run over them) in the world of LIMBO. Each egg is tied to a special achievement. You might have found some eggs on your first playthrough, but some of them are quite difficult to find and it's easy to miss them. But that's no problem as you can jump back to each chapter after beating the game.

By collecting all eggs you also unlock the entrance to a secret level which adds around 15-20 minutes additional playtime and a very special challenge to the game, it plays in almost complete darkness. For more information please view my other guide.

http://steamproxy.net/sharedfiles/filedetails/?id=113463434
Here's a list of all chapters with hidden eggs and the corresponging achievement:

• Chapter 1: Wrong Way
  
• Chapter 4: Altitude is Attitude
  
• Chapter 19: It's Stuck
  
• Chapter 24: Urban Exploration
  
• Chapter 26: Alone in the Dark
  
• Chapter 27: Climbing the Cog
  
• Chapter 31/ End of 30: Backtracking
  
• Chapter 32: Guided by Sparks
  
• Chapter 34: Under Ground
  
• Chapter 38: Going Up

Note: The Backtracking egg is officially located at the end of chapter 30 but it's easier to go backwards from 31 without the brainslug. And please be aware that this is a guide for the pc version, earlier console versions have different chapter numbers.