Quick Selection and Magic Wand Tools (W)
Both the Quick Selection and Magic Wand tools make selections based on the color sampled. When using the Quick Selection tool, checking Auto-Enhance can help reduce rough edges of the selection in transitional areas. For more control after making a selection, (when using either Quick Selection or the Magic Wand tool), select Refine Edge from the Options bar and use the dialog box to further fine tune your selection.
Both tools will also sample similar colors on multiple layers - simply check the "Sample All Layers" feature in the Options bar.