If you have't ever worked with a dark photo, you're quite lucky; I'll buy you a lotto ticket if you split the money with me. However, just in case you've got inhuman luck, I'll use my own personal experience as an example.
I have a $200 digital camera from Kodak. I recently went on vacation and stopped by a forest of redwoods and took some pictures. The forest isn't extremely dark, but the trees aren't exactly radiating natural light, either. So a few of my pictures came out a little dark. Photoshop to the rescue.
Don't assume that because your image is dark, that all the detail is gone. Something that looks like a black blob can actually have a lot of detail - it might just not look like it because the human eye has a hard time distinguishing details in the dark. Look at Figure-1, which shows the before and after photos.
Most people try to use the brighten and contrast tools to pull out those dark details. A standard technique to do this is to open up the Brightness & Contrast popup (Image >> Adjustments >> Brightness/Contrast...), push the brightness up, and then increase the contrast. Unfortunately, this technique isn't usually as effective as another Photoshop tool called Levels (Image >> Adjustments >> Levels... or Ctrl+L).
In Figure-1, I didn't use the brighten or contrast tools. All I did was open up the Levels popup, shown in Figure-2:
... and I took that middle slider (circled in red at the bottom), and slowly pulled it to the left until the picture looked bright enough. That's all there was to it!
The nice thing about this method is that it only brightens up parts of the image that are dark. Notice the sun's rays on the tree trunk before and after - they're almost identical, while the darkened details of the tree trunk came out.
Keep in mind that there IS a limit to the amount of detail that you can bring out. If you pull the slider too far, your image will begin to lose details.
If you drag the left slider to the right, you're making the dark parts of the image ("Shadows") even darker. If you drag the right slider to the left, you're making the lighter parts of the image ("Highlights") even lighter. Tinkering with these sliders can produce some nice, dramatic lighting effects.
For example, I took a shot of a large, dimly-lit painting that was behind some shadowed objects, and it was dramatic already. However, by intensifying the Highlights, I was able to make the painting seem like it was almost glowing, which created a beautiful effect. Just keep experimenting with this tool on different images - it's a tool that I use frequently for different purposes, but can make your photo-editing life a lot easier.