This very simple, yet effective, lighting technique comes from a class taught at Photoshop World by Eddie Tapp. Among his many talents, Eddie comes up with cool techniques that are easy to do but can create a powerful result.
Here's the image I'll use for this technique.
Click on the Create New Adjustment Layer icon at the bottom of the Layers palette, and add a Curves adjustment layer from the pop-up menu. Click on the top-right point of the curve and drag it straight down until the Output number is between 70 and 100.
Here's the result of my Curves adjustment layer.
Press B to get the Brush tool and choose a very large, soft-edged brush. Make sure your Foreground is set to black and, in the Options Bar, set the Opacity of the brush to around 5060%. Click once on the image to hide the effects of the adjustment layer (and add the lighting effect). In this example, I then changed the brush opacity to 20%, moved the position of the brush slightly, and clicked again. Here's the result and what my layer mask looked like.
Here, I changed the blend mode of the adjustment layer to Multiply, which intensiffes the darker areas.
Variation 1: Multiply
Rather than using a brush to mask the effects of the Curves adjustment layer, you can also use a selection. Here, I used the Lasso tool (L) to make a rough selection of the area I wanted to light, and pressed Option-Delete (PC: Alt-Backspace) to fill it with black. After deselecting, I used Filter>Blur>Gaussian Blur to create the look I wanted by experimenting with different amounts of blurring.