The Package Explorer utility that Wouter van Vugt has uploaded to OpenXmlDeveloper is a great starting point for building a generalized Open XML utility. It provides the ability to open an Open XML package and browse through the relationships and parts, and is developed in C# as a Visual Studio WinForm application.

You can now contribute to this utility on Codeplex, Microsoft's newest open source community. Here's the link to the Codeplex project.

Sign up to become a project member on Codeplex and help turn the Package Explorer into the Package Studio of your dreams! Wouter has posted some ideas for enhancements on the Codeplex project page, and you can contribute your ideas, enhancements, and source-code modifications there.