Free refills on drinks are common in most restaurants in America, both in fast food and in sit down restaurants. There are exceptions but unless it is specifically signposted you can expect to get your drink refilled without further charge. This applies only to sodas/pop/soft drinks, tea, coffee and iced tea and not other drinks like milk, juice and alcoholic beverages. I had become so used to getting free refills on drinks in America that I totally forgot to tell my family about it. I felt bad but I was glad that my sister spoke up so we could fix it. Often it is printed on the menu that the refills are free but its normally in small print so unless you were looking for it you could easily miss it.
Drink refills are normally not free in Australia unless specifically posted otherwise. It is traditional to pay for each drink that is brought to you so if you get 5 glasses of Coca-Cola you will be paying about $2-4 per glass which adds up. I do remember there was one buffet in town when I was a kid that had free refills and that place was one of my favourites as I didn't have to worry about how much I was costing my parents. Everywhere else we normally had just one or two glasses and we drank slowly so we didn't run out too soon.
Free refills are one of the many things I love about the American culture. I will miss having free refills when visiting friends and family in Australia but unless I am mistaken water is still free so I should be ok. Is there any unexpected customs you have noticed when visiting another country?