Organ meats- liver and kidney from any animal, or even beef sweetbreads or lungs if you can get them. Be careful not to give too much organ meat at first as it will result in bad runny poop, I give a small amount 1-2 times a week.
http://rawfeddogs.net/RecipePhotos/10,
http://rawfeddogs.net/RecipePhotos/21 http://rawfeddogs.net/RecipePhoto/27/0 http://rawfeddogs.net/RecipePhotos/44
BTW, heart is GREAT but it is classes as a muscle meat, not organ.