The classiest drop shadows are rarely black; instead, they pick up a darker color from the image. In Photoshop, you can add a drop shadow to a layer by clicking the fx button at the bottom of the Layers panel and choose Drop Shadow.
In the Layer Style dialog box, click the color swatch toward the top right and Photoshop summons the Color Picker. To snatch a color that already lives in the image, mouse over to it and, when your cursor turns into an eyedropper, click once; your shadow takes on that color.
(Tip courtesy of Lesa Snider at www.photolesa.com.)