Open XML documents are simply ZIP packages, but they follow a standardized architecture called the Open Packaging Convention. You can work with OPC packages (as used by Open XML, XPS, and various other formats) using simple tools such as WINZIP and Notepad, but if you're trying to be productive you'll find that approach tedious.
If you want a better tool for this kind of work, and one that can also validate the parts within an OPC package or handle other common Open XML development chores, look no further: Package Explorer is what you want, and it's an open-source project on Codeplex so you can get it with full source code.
Wouter Van Vugt is the architect of Package Explorer, and Doug Mahugh sat down with Wouter in Building 20 recently to discuss the status of the program and see a demonstration of some of its recently added new functionality. If you're doing Open XML development, this utility is a must-have for your Open XML toolbox!