First off, supplements are not required for your dog to have a complete diet.  These items are purely to provide additional fat, vitamins, minerals and nutrients.  If you are feeding a complete diet (meat, bone and organ) you are more than likely fine and can give any of the items below as you wish, and in small doses.

Greek Yogurt- yogurt made in the style without gelatin

Alfalfa- powdered or tablet

Kelp- ground

Digestive Enzymes- powdered

Apple Cider Vinegar- liquid

Brewer's or Digestive Yeast- powdered

Eggs- with some of the shell

Vitamin C- liquid, powder, or chewable

Fish Oil- liquid or capsules

Flax Seed Oil- liquid or capsules

Cod Liver Oil- liquid or capsules