Protexin for my horse - first used it after my horse colicked. Then continued to use it when she was likely to be stressed. Now she's older, I find the changing seasons (hot for a few days then much cooler, really dry then hissing down with rain) seem to take a toll on her digestive system so she's on Protexin permanently at maintenance dose.
For hind gut issues I have a homemade mix of brewers yeast, yea sacc and spearmint. It clears up loose droppings in my gelding within two days.
For acid relief, I've used Acid Ease in an ulcer horse.