Comparable to a previous post using DirectShow Filter Manager, GSpot helps shine some light on mysterious codecs at work. While DSFM will show you what codecs are installed on your machine and what priority they are taking precedence for different file types, GSpot can show you advanced information about the codecs installed on your machine and those used to encode a file.
Just browse to a file and find out all sorts of information regarding the process of rendering the video for your viewing pleasure. A fine troubleshooting tool.