Just a small tutorial to fix underexposed pics with GIMP.
My under-exposed pic..
Open the pic with GIMP 2.2
if you don't see the "Layers" window around.. you need to bring it out.
Select "Dialogs" -> "Layers" or press "Ctrl L" to get it out.
click on the 'Duplicate' icon (circled in red) see picture below.
You should see a duplicate picture layer called 'Background copy'
Click on the words "background copy" to hilight it.
Next click on the drop down box for "Mode: Normal" (circled in blue) and select "Screen" (boxed in red)
finally, Select "Layer"->"Merge Down" and save your picture.
If you find it too bright, you can adjust the intensity of 'brightening' by adjust the 'Opacity' below the "Mode:" box.
For mild (note the word mild) over exposure, you can try to set the blending Mode to Multiply instead of Screen. For most overexposed pics, the details are lost and cannot be recovered most of the time (unless you're shooting raw).
Hope this helps to promote new users to GIMP. I'm still in the process of discovering more of GIMP's features.