VLC Media PLayer was recommended to those of us taking his Video course by Craig Daniels, and it has been a great tool for making good video part of our toolkit.
It is simple to use, plays pretty much everything in the way of available video formats, and runs on Windows or Macs (or Linux if you're a geek). It's excellent for viewing the raw footage I have taken for making a video, and I use it for identifying which clips and which parts of those clips I want to use for production, It's much quicker and easier than trying to do that in my video editing software. It's also great as a general purpose viewer.
I would recommend that you download it from the official site - other sites that offer it will sometimes try to add unwanted programs like Yahoo toolbar or something similarly obnoxious.