Epsom is used to add sulfate and magnesium ions to brewing water. It is often used at half the levels of Gypsum. 1 gram in 1 gallon changes the salt levels by 103 ppm sulfate, 26 ppm Magnesium and adds 108 ppm to the hardness.
Promash is a useful utility to calculate water salts.
Epsom salt is also ideal as a nutrient source for growing hop plants.
TIP: If you're really sore after brewing and your hands or feet are aching, try adding 2oz of epsom salt to one gallon of hot water and soak for ten minutes. Temporarily relieves pain.