Balut is a fertilized egg embryo that is typically aged for 16-21 days (depending on which country you eat it in) then boiled like a hard boiled egg and eaten. It is supremely popular in the Philippines and Vietnam.
I have encountered it often in those countries but just couldn't get myself to try it. My western palate just can't stomach this.
It is such a difficult concept for us that it was even a Fear Factor even