VideoInspector is a handy tool that tells you everything you might want to know about a video file.
Just open a video and the program will tell you its duration, resolution, frame and bit rates, for instance. You'll also see confirmation of the video format, and VideoInspector will let you know whether the file appears to be valid, or not. (If it isn't then the video may be corrupted, good to know if you're wondering why it won't play).
VideoInspector will also tell you which video and audio codecs the file requires, and whether you have them installed, as well as providing a download link if you don't.
And extra tools include the ability to list all your installed video and audio codecs, along with an option to burn your selected videos to CD or DVD. (This requires a free CopyToDVD add-on, but the program will automatically download that for you if you don't have it already.)
Please note, while VideoInspector is free, it will install various browser add-ons and other adware during the installation process if you accept the default settings. Be sure to check your setup options carefully, and clear the boxes next to anything you don't want to install.