Ok, to start off, we obviously need an underexposed photo. I will be using this photo below, if you want to follow this tutorial, you are welcome to save this photo and use it for this tutorial.

In the days before photoshop CS, to fix this photo, we would need to use the Brightness and Contrast adjustments, so lets try that and see how far we go...

Go to Image » Adjustment » Brightness & Contrast.
Now I would move the brightness and contrast sliders over to the right to around about +30 - +35 - as this would give the best result, any more than this will make the image too bright and washed out.

Here is the result

Not bad i reckon, but if you are as picky as I am, then this is not good enough, the image appears to be washed out a little, and the monitor on the left is still too dark. So scrap all that and lets explore a new feature in photoshop CS

With the original image, go to Image » Adjustment » Shadow/Highlight.

Instantly, this adjustment will improve your image, without overexposing it. You can slide the Shadows slider over to 100% to really brighten it up, while still maintaining the image's exposure. If you did this using the Brightness/Contrast option above, your image will render unrecognizable.

So there you go, a super fast and easy way to fix your underexposed photo.