The more capable image-editing programs (Photoshop, The GIMP, etc.) will also let you adjust the levels in an image pretty easily. Open up the Levels dialog (in The GIMP, it\'s Layer | Levels...) and drag the left-hand slider on the histogram (circled in red) to the right. A good starting point is where the histogram data starts rising.
The sliders under the histogram control the range of luminance; the left-most slider sets the point where everything is completely black (well, actually the value of the left \"Output Levels\" slider), and the right-most slider has full luminance (or the value of the right \"Output Levels\" slider). Everything between the sliders is mapped accordingly.
The middle slider is used as a gamma control. You can usually also affect RGB levels individually as well as the luminance value (in The GIMP, just drop down the \"Value\" box to select another channel).
This tool is really helpful in making washed-out photographs look better, too.
