I use Purina BMZ - it's a lot like Farriers formula, but way less expensive. I also add soybeans to the ration and it's made a huge difference in my filly. NuImage hoof and coat support is in a soy base, so I figured adding soy would not hurt!
I feed the appropriate weight in a pelleted ration, free choice hay, freechoice salt, the BMZ which is a tiny scoop, and about 150 grams of soybeans. Many supplements are soy based now - it's great for coat and hoof. If you have an overweight horse you may want to think twice, though!
My vet told me about MSM and it has worked miracles. My draft had really bad feet despite regular trims. About five to six months after I started using the MSM me and the farrier started noticing how much better Solon's feet were. A few other people at the barn started using it and it's really worked wonders. I swear by it.
I have had people tell me MSM isn't supposed to have that effect on hooves but I've seen a few articles over the past year indicating that it indeed can have a good effect on them.