I took a few shots today where I COMPLETELY blew the exposure and the pictures were nearly completely black. I noticed it and changed my settings, but lost about 8 shots before I noticed. A lot of people will tell you that you can't recover this type of picture except in RAW. That is not entirely true. Here is an example. This picture was shot with a Canon 1DMk2N (8MP) and using JPEG Fine.

Now, here is the corrected one... What I did was open the image in Photoshop first. Then I cropped it the way I wanted. After cropping, I switched to Mode > 16 Bit. This improves the results of corrections, but needs to be changed back to 8 Bit before saving as a new JPG file. I then went into Levels, and moved the right and left sliders to toward the middle until I got to an area of the histogram that had data (I essentially wiped out the flat areas on the left and right). Then I took the middle slider and went +/- 0.2 until I found the ideal spot to bring out the midtones to be pleasing. I would have been good to run this through some noise removal at this point, but I didn't. I sized and sharpened and this is the corrected image.

So don't throw away those dark images if they are important. You can definitely recover a lot of the detail.