Windows only: MediaTab is a free program that provides tag and other technical information about audio and video files, like MediaInfo, but is integrated into Windows Explorer, so you don't need to open another application to get these details.
To view an mp3, avi or other multimedia file's detailed media properties, simply right-click on the file in Windows Explorer and go to Properties (or even quicker: hold down the ALT key and double-click), then go to the MediaTab tab. Press the analyse button and you can view general file information, codecs, bitrates, resolutions, durations and much more in either text or tree view. You can also export the info to text or HTML and/or copy to the clipboard from the MediaTab.
All in all, a handy utility for quickly finding out everything about a media file, from within Windows Explorer.