My barn swears by Leaps and Bounds. I just started my own horse on it and other horses at my barn have great feet after having been on it long enough for hte hoof to grow out (a year ish). Once a visible growth line is there I'm going to post updates.
Yes, anything that has Biotin in it. You get biotin supplements by themselves. Biotin makes the nails grow stronger and faster, and helps with mane and tail growth, much like with our own fingernails and hair =)
Agreed, make sure the horse's diet is complete first, with very low sugar/starch content (no grains or grain products, or very, very low amounts), then you can consider adding a good hoof supplement. You need to look for one that also has lysine and methionine in it, the two amino acids that help the body absorb and utilize the biotin. I have had great luck with Source Focus HF and Next Level Hoof Liquid.