Arborvitae but they can be expensive and if you get small ones they can take years to grow full sized. My sister in law did this because her house was on a corner lot and a major road that was loud. She loved the outcome. http://arborday.org/trees/treeGuide/tre ... .cfm?ID=78
Bamboo is pretty cheap and easy but its invasive and will go wild if it isn't constantly maintained.