AltaLux is an image processing technology that can significantly enhance the quality of images and videos with poor lighting conditions.
AltaLux/Demo is a Windows sample application that lets you enhance the quality of JPEG images for free (download it! from the download area). For even better usability, I strongly recommend using AltaPixShare as it has a vastly better interface.
The animation above shows how the AltaLux filter performs on a real photo, ranging from left (no enhancement) to right (max level of enhancement). On the original photo the palace is underexposed, it appears dark as the sky in the background is much clearer; increasing the amount of enhancement to an intermediate level takes the palace out of the dark, exposing its facade; moving the enhancement to max is too much of a good thing, as previously hidden details pop out of the screen but the scene does not appear natural at all.
Update: the new 2.5 version includes an additional setting (Scale) that changes how each zone of the image influences nearby ones.Click here for a visual explanation of how it affects filter's results.