There are many low calorie options at starbucks, in fact I have started drinking there most everyday.
a grande americano is 15 calories, I usually add some sugar free flavoured syrup and skim milk
(iced is same amount of calories)
a grande non fat misto is 70 again I usually add some sugar free falvoured syrup (it is half way between coffee and latte, they add steamed milk)
There are many other options, look at their site and play around with nutritional information. I always use the sugar free syrup instead of splenda or something because it makes it feel more like a treat
Some of their frap drinks (in light version) and pretty low too, a tall light vanilla bean is 120 and just like ice cream!