Back when we were working heavily on the OpenXML file format developer tools, many of us were using zip and Notepad to edit the OpenXML files. Tom Underhill from the Office graphics team made all our lives easier with a VS package for editing OpenXML files. I used it quite a bit during the OpenXML code snippet development (you might have even seen a quick glipse of it in a demo somewhere). There are other Package file editors available, (like Wouter's), but I like this one mainly because its integrated in VS. (Where I spend much of my time these days).
Andrew Whitechapel and the VSTO folks got a shipping vehicle for Tom's OpenXML Package Editor. Along with a number of other VSTO and VSTA Powertools, the VS-integrated Package editor is available here.
Here's a list of features you'll find in the VSTO Powertool Package Editor:
Andrew has all the details on his blog.