If you're doing much Open XML development, you probably sometimes need to compare two versions of a document and look closely at the differences. Stephane Rodriguez's diffopc tool is a utility program that makes short work of this sort of a task.
Diffopc takes any two OPC (Open Packaging Convention) files as inputs (e.g., Open XML documents or XPS documents), and displayes a well-organized summary of the differences in a web browser. You can look at the parts that have changed, or drill down into a part and see the specific differences in the XML markup.
Download or read more about diffopc here.