I've just finished playing through Limbo for the first time, and now I'm going to be starting again and collecting the remainder of the eggs that I missed. Most of the eggs in this game seem to be attached to achievements... I say most because I picked one up toward the end of the game that wasn't attached to an achievement. To that end... I'm curious how many other extra eggs there are in this game, and what might happen if you find them all.  
 
The egg I found was near the end where the anti-grav zones start popping up. There's an elevator, some machine guns, and a switch that will flip the gravity (right above where you get the "underground" achievement. I won't give away how to get to it, but this extra egg is somewhere in this area.  
 
Has anyone else spotted extra eggs lying around anywhere in the game?

Yes, I found one at the beginning of chapter 17.
 
When you start riding the giant cog up, instead of jumping off to the left, stay on it all the way to the top until it stops, then jump to the right and walk a little bit. There will be an egg in the darkness. At first I thought this was the one for the 'Climbing the Cog' achievement, and wondered why I didn't get it, but it turned out that was somewhere else.
 
At 99% now, I know I still need the one you just described, but I'm really curious how the current number 1 guy got 105%. I guess the hunt is on!

Interesting... so there are at least the two, and... maybe if it's 105% there could be as many as... 5?

Oh snap! Look at this!

There is actually one more after that one. Go all the way to the right of the light and climb down and you can keep going. Drop down over another part after you see the egg and pull out a box then drop back inside.

Yeah, I changed the video to the new one now. That guy already has every egg up to 108% completion on video as of this writing.

Make that 109%! 
 
Yes, those are my videos. :) 

Up to 111% now for top spot... 103% will put you in the top 500. 
 
Update: If you want to see some more... head over to this link...