Many people want to save rain water but do not have a gutter system. Certainly a gutter system is a natural way to harvest the rain that washes over the roof as it directs it into a concentrated location and fills your barrel quite quickly (most barrels fill in less than 1/4″ of rain). However there are still ways to collect rain water with out gutters. Look at your house the next time it rains and identify a location where water flows off in a concentrated stream. Some times there is a pitch or valley in the roof where this happens, or perhaps a joint where two roof planes connect and place your barrel beneath it. Some people use rain chains to help direct the water. One note of caution is to be careful attaching metal to wood, as it will rot.
It does not make sense to put a free standing barrel out in the open to collect rain water. It will fill inch for inch. In New England that would take about a year, while a rain barrel collecting off a roof will fill in a storm or two.