You fill the mugs yourself from the beverage dispensers. Mainly sodas and iced tea but you can also get coffee and hot chocolate although my wife tried the coffee once and felt it was particularily bad in the plastic mug.
Last year at the Poly, I remember Diet Coke (drank a few hundred litres of it), Coke, Cherry Coke, Coke Zero, Lemonade, Root Beer, Orange etc.