This is a pair of Photoshop actions I created some time ago at the request of a friend who was sick of their screenshots always being too dark.

Installation:

This file must go into your Photoshop Actions folder. Usual locations can be:
C:\Program Files\Adobe\Photoshop ( version ) \Presets\Photoshop Actions
C:\Program Files ( x86 ) \Adobe\Photoshop ( version ) \Presets\Photoshop Actions

This was created with Photoshop CS2. It will work on later versions, but I'm pretty sure that it will not work on older versions due to Adobe writing in incompatibilities.

After installing, Open Photoshop and click the Actions options button ( little circle with a triangle in it ) .
Click Load Actions and then select Dark Screenshot Cure.
It will be at the bottom of your Actions list after loading.

There are two different actions which are completely independent of each other.
Dark SS 1 will usually do the trick for brightening up an image. The original image is left unedited underneath the new layers which comprise the work of the action.
These can be further edited.
Dark SS 2 takes a slightly different approach, and can be edited after as well. The difference being that with Dark SS 1, you can merge the edited layers together. They are not reliant on the original image after the action has been run. With Dark SS 2, the new layers are dependent on the original image under them.

Requires Photoshop CS2 or above.
Does it work on GIMP? I have no idea. I don't use GIMP.

Have fun!

Credits:
Adobe -  for continuing development on Photoshop
Mr. Dave - for the dark SS 1 and 2 actions.
Mr. T.