Butterbeer
Very popular drink served cold in bottles or hot by the mug ("foaming tankards of hot butterbeer") at the Three Broomsticks. House-elves can get drunk on butterbeer, but it doesn't have that effect on humans (GF21).
Available at the Hog's Head for two Sickles a bottle (OP16).
How is butterbeer made and what does it taste like? When asked about this by "Bon Appetit" magazine, JKR responded: "I made it up. I imagine it to taste a little bit like less-sickly butterscotch."
