Shadows and Highlights, the Easy Way
Unfortunately, Adobe hasn't seen fit to let us use it on an adjustment layer, but the aptly named Shadow/Highlight adjustment in Photoshop is definitely the easiest way to bring out shadow detail, tone down reflective highlight glare and make other common tonal adjustments. Here's how it works.
First, open an image that needs some work. Now make a copy of your background layer by dragging its layer thumbnail from the Layers palette to the New Layer icon at the bottom of the Layers palette. Since Shadow/Highlight doesn't work as an adjustment layer, having a copy will let you put some of the original back later if you find you prefer it. All your adjustments will be done to the copied layer. You can ignore the original background layer for now.
Now go to Image >> Adjustments >> Shadow/Highlight. The basic dialog has only two sliders, one for Shadows and one for Highlights. If that seems too simple, it is. Click on "Show More Options" to access to the real power of Shadow/Highlight. This will give you three sliders each for Shadows and Highlights plus several bonus sliders and controls in a section labeled "Adjustments." More sliders means more control.
You probably noticed that as soon as you opened the Shadow/Highlight dialog that your image got a good deal brighter somewhat flat looking. Don't be alarmed. The default settings for Shadow/Highlight are a bit too aggressive but thankfully can be changed. If you find some settings that come closer to what you generally end up with, you can click on "Save As Defaults" so you'll start there next time rather than the ones Adobe gave you as part of installing Photoshop.
As for what to do with all those sliders, let's start with the Shadows both since it's at the top of the dialog and also because it's the one that most often makes the most difference. Grab the Shadows Amount slider and swing it from one end to the other to get a feel for the range of what is possible. Generally, a little bit will improve a dark image whereas too much is way too much. If you set the Amount too high you likely start to notice halos and other artifacts starting to form. Setting the Shadows Amount too high in an attempt to fix an underexposed image will also tend to make any noise more evident. Let go of the slider when the image looks the way you want it. The other two Shadows sliders will let you fine tune things a bit but you need to be in the right ballpark.
The Shadows Tonal Width slider controls the range of tones that will be affected by the Amount. Lowering it will mean that your changes will only affect the deepest shadows. Raising it means that brighter portions of the image, closer to the midtones, will also be affected. Raising or lowering the Radius tells Photoshop whether or not to consider neighboring pixels when determining whether each pixel qualifies for being included in the effect. A low radius constrains changes to only those pixels themselves that are within the Tonal Width selected while choosing higher radius values averages in a proportionally greater number of surrounding pixels when making that determination. Both of these sliders can help you keep the effect looking realistic and can help minimize any halos the Amount may have caused.
The Highlights sliders work similarly to the Shadows adjustments in many ways. The Highlight Amount slider may at first seem somewhat counterintuitive though since raising it makes the image darker rather than brighter. Think of it this way though: both Shadows and Highlights amounts work to pull their respective portion of the tonal spectrum closer to middle tone. Thus, raising the Highlights Amount darkens the image since that's the direction we have to go in order to get closer to midtone.
Highlight adjustment Amount starts out at zero meaning that the highlights won't be changed which works just fine for many images but not all. You can't fix an overexposed image with the Highlight Amount, but if you have an image with a slight reflective sheen on some areas raising it may be just what you need to restore detail.
At the bottom of the Shadows/Highlights window is a section generically labeled as "Adjustments." Not that the sliders in the Shadows and Highlights sections aren't also adjustments of course, but they had to call this bottom section something and "Miscellaneous doo-dads" probably didn't test market very well.
Color Correction allows you to tweak the saturation of the areas being affected. When editing and RGB image, setting the Shadows Amount too high can result in a fairly flat looking image. Color doesn't expose well in the deep shadows and adding a bit of extra color back into these areas with the Color Correction slider can help make the image look more like it was exposed correctly to begin with. If your Amount settings are more modest though, the effect of the Color Correction slider will be less obvious as well as less needed.
Midtone Contrast allows you to fine tune the contrast of an image without affecting the shadow and highlight detail you have worked so hard to restore. Raising this has a similar effect to creating a S-curve in the Curves dialog, making your image have greater apparent contrast. Raising it too much will start to darken your shadows and brighten your highlights so there is some interplay between this and the basic Amount settings in extreme cases.
The Black and White Clip settings allow you to control the amount of clipping on each end of the tonal spectrum. Values less than the Black Clip point will become pure shadow in your edited image, preventing the Shadow Amount slider from affecting really deep shadows. An image can look somewhat unnatural if it has no shadows at all, but the default of 0.01 should be fine unless you have had to do something extreme with the Amount. The White Clip setting works similarly. Anything closer than this to pure white will become pure white and the rest of the image tones redistributed to make up the difference. Since burned out highlights are almost never a good thing, I don't think I've ever felt compelled to change the White Clip value. You can tell Adobe assumed you wouldn't need to use these two all that often since they didn't even provide sliders for them. If your cursor is positioned in the input box for either though you can use the up and down arrow keys on your keyboard to adjust them almost as if you had a slider. You can do this with any numeric input box in Photoshop of course, but I tend to grab the slider with my mouse if one is provided. Keep in mind that these Clip settings are in percent such that a Black Clip of 50 means middle tone, not 50 out of the possible 255 for an 8-bit channel value.
If, after adjusting the Shadows/Highlights settings you find that you want some of your original image back again, just add a layer mask to the duplicated layer that you adjusted and paint away with black or gray hide your edits as needed. Painting on the mask with white will reveal your edits once again if you change your mind back later. Lowering the opacity of your duplicated layer will let you lessen the effect over the entire image. Even though your Highlight/Shadow adjustments aren't on an adjustment layer, you can retain at least some control with the effect on its own layer.
Before Shadows/Highlights adjustment
After Shadows/Highlights adjustment
You can pretty much do everything Shadows/Highlights can via other tools in Photoshop, but having all this power in a single, fairly simple dialog can make your life that much easier.
By the way, why Adobe named the menu option "Shadow/Highlight" but titled the resulting dialog window "Shadows/Highlights" is beyond me. One is singular and the other plural. If they read my website, some proofreader at Adobe must be hanging his head in shame right about now.