Yoghurt and butter are the main things that come to mind for sour milk. Some cakes and biscuits use buttermilk in the recipes as well. I wouldn't use it for drinking or cereals though as the taste is not nice.
There are numerous of recipes you could try, if you don't want to waste your money. Could have yogurt and lot's of other dairy products but i don't think so there would be any way to turn it back to fresh milk.