The old way to do shadows (before drop shadows were added as a layer effect) was basically to do what glen said: one created a shape from the original item with pen tool, lasso, etc. One then created a new layer and filled this shape with black. Guassian blur was added. The shape was moved to offset it slightly from the original and then the layer was placed under the original. The shadow layer was generally set to multiply mode and the opacity dropped to 30-50%. Sometimes one further masked the shadow by adding a layer mask and filling with a gradient fill.