Creativepro.com has an excellent excerpt from The Photographer's Photoshop CS2 Companion by Mark S. Johnson about digitally re-creating a graduated neutral density filter. Graduated ND filters are used by nature and landscape photographers to balance the exposure of a light sky and dark foreground. Now we have ways to get the same balanced exposure digitally.
The excerpt, found here in PDF form, explains how to use a layer mask and two different exposures of the same image to balance out the lighting in the scene.
This method is thorough but does require a tripod and two frames taken at different exposure settings to give you one for the ground and one for the sky. I have used a similar method of layer masking using only one picture, duplicating the layer and adjusting it to give me my "second exposure". This only requires one initial exposure and a few minutes in Photoshop, the downside to this method being higher noise levels in your darker exposure if the contrast in your image is too high.
_ —Matthew Panzarino
Image: Mark S. Johnson