Biotin Plus (from Paragon)
I've had great success with Biotin Plus in getting my horse's feet in better condition. It has a much higher concentration of biotin than most other hoof-growth supplements, and is great for a shiny haircoat and mane-&-tail growth as an added plus. I supplement it with DL Methionine, which is necessary for the horse to utilize the biotin. Biotin Plus is easy to feed (you just throw it in with some other goodies), because it has a pleasant, spicy smell most horses will readily eat.