MPEG Streamclip 1.2 (Windows, Freeware, 542KB) is a powerful high-quality video converter, player, editor for MPEG, QuickTime, transport streams, iPod. And now it is a DivX editor and encoding machine, and even a movie downloader.
Mac OS X version also available.
I sometimes need to take videos with my digital camera, and more often than not, the videos are too dark. MPEG Streamclip lets me adjust the brightness (among other things) of the original video, and create a brighter new video file.