MARTA is the city transportation system in Atlanta. The CityPass allows access to several Atlanta attractions at a fraction of the cost of admission. The CityPass is $65, but admission to the Aquarium only is $29. Buying the CityPass ends up saving you a great deal of money!
The best things in life are free, but in Atlanta, the best will cost you. So far, it seems that MARTA and the CityPass will give the best bang for your buck.