Minecraft wiki entry for Ranching, associated text: Cows additionally drop leather. Both cows and mooshrooms can be milked: cows can be milked with a bucket for milk, and mooshrooms can be "milked" using a bowl to get mushroom stew. Mooshrooms can also be sheared for 3–5 red mushrooms, although this will turn them into normal Cows.
Chickens additionally drop feathers. Eggs can be harvested from chicken farms, and can be used in addition to chicken breeding to speed population growth, as they have a 1⁄8th chance of producing (at least one) baby chicken when thrown.
Sheep drop one wool when killed, but it is more productive to shear them, as they drop 1–3 wool. One dye can be used to color their wool before shearing or breeding, which is more efficient than dying the single wool after shearing. Dyed sheep produce offspring of their current color; if the two sheep are different colors, the color of the baby sheep will be a mix of its parent's colors. Ex: blue + white = light blue sheep
Rabbits may drop rabbit hide upon death, which can be used to create leather if you are short on cows. They also have a small chance of dropping rabbit's foot, which can be used to make a Potion of leaping.