"A histogram can represent any number of things, since its sole purpose is to graphically summarize the distribution of a single-variable set of data. Each specific use targets some different features of histogram graphs, and when it boils down to image analysis, histograms are the de facto standard."
Included with a short, five-page article describing the concept of histogram equalization is a Java 1.4.2 implementation. It features two methods of image equalization: cumulative and normalized.
The source code is included, but you may also simply just run the executable. I didn't focus on equalizing color images, although I do discuss a solution in the paper. | 
Please contact me prior to distributing any files.