I am not an expert on editing, but I have found a very nice freeware video editing tool, called VirtuaDub. It is very comprehensive. The only thing about it is that the user requiers some technical knowledge. You can become familiar with it in a short time, just poking around in the menus. The user interface is not complicated, but it is not as easy as 'press this button and it does it all for you.'

The author has made it possible to install filters for various editing reasons. He has a link to another site that has filters for a few programs, VirtualDub included. When video is captured and there is a darkening problem, the filter author has written two filters for Histograms. The filters balance dark and light areas, and can regulate brightness to a certain extent.

The author said that the filters don't work for every type of circumstance, but they might help you. It's worth it to mention.

http://www.virtualdub.org <--- has the editor

http://neuron2.net/ <--- has extra filters (many types)