One of the most common problems when taking photographs is that a picture can be both underexposed and overexposed in places. In the picture below Pearl was too dark in the foreground and the background were totally bleached out (look at the sky and sign).
Modern SLR cameras that can take photos in RAW format hold an unprocessed copy of the image which is normally lost the second you convert the picture into a viewable/printable format such as JPG/GIF. RAW editors are normally supplied with SLR cameras that allow broad brush adjustment of the picture, but this is normally little more than the ability to undo/redo some of the settings that you made when you took the picture (such as white balance) and a contrast adjustment.
An add-on to photoshop provides some better tools for editing the RAW picture before it gets added into photoshop but this suffers from being destructive and a one way process.
Lightzone a $99 photo editing tool allows you to apply changes to the picture which are completely none destructive (the original picture is untouched) and all changes are made on the fly and can be undone/modified.
You can selectively pick portions of the picture to apply edits to and this can be based on regions or colour ranges or combinations of the both. For example I was able to fence Pearl and adjust the underexposed areas, and select the rest of the picture and adjust the overexposed areas. I was then also able to pick the blue of the sky and selectively adjust the sky contrast and hue without affecting blues elsewhere in the picture.
More lightzone info can be found here http://www.lightcrafts.com/lightzone/
A Flickr Pool for Lightzone users here http://www.flickr.com/groups/lightzone/pool/