Seeking free and simple way to brighten a dark video
Windows:
If you are on windows 7/Vista, you should have Windows Live Movie Maker. You can open your video in that, go to the Visual Effects tab, and select the brightness button. Then you have to re-encode your video. This will decrease the quality.

Free Windows Live Movie Maker

The problem with Windows Live Movie Maker is that there are limited effects and no way to create custom effects. However, Windows Movie Maker from Vista (i.e. WMM 6.0) does support custom effects, etc.

You can download WMM 6.0 for Windows 7 here.

Blaine also provides an extensive list of add on effects here. Which includes contrast effects to increase the contrast.



Mac:
If you have a MAC and iMovie, I found this link. You hover over the thumbnail and a "sun" symbol should show up, clicking that should bring up color controls.



Hopefully one of those will work. If not, you'll have to dive into the world of opensource video editing tools, which is a nightmare.