I received this Photoshop tip in my Bravenet Webmaster Tips and Tricks newsletter and thought it was really good. I had never tried using the Blend mode options before.
by Jake Redekop
"Layer blend modes are a powerful, but often overlooked, feature of Photoshop. This month we're going to use layer blend modes to correct under or over exposures in your digital photograph collection.
We'll begin with overexposures. The first thing you need to do is duplicate the layer that holds your overexposed photograph. To do this, right click on the layer and chose “Duplicate Layer” from the drop down menu.
You should now have two instances of your photograph. Choose the topmost layer and switch the blending mode to 'Multiply' (located at the top of the layers palette). If you like you results, you are done!If your photo is still a little light, keep duplicating the topmost layer until you are satisfied; however, if the picture is too dark now, lower the opacity of the top layer until you get the desired color saturation. For the image below, I used two duplicates with the second duplicate set to 15% opacity.
To correct underexposed photographs, follow the same steps as above except use the "Screen" Blend Mode to brighten the image."