Developers working with Microsoft Office files can now freely download he specifications for both the Office 2007 Open Office XML (OOXML) Formats and the older Office 97-2003 Binary File Formats (BIFF), for Word/Excel/PowerPoint.  Microsoft has agreed to cover both sets of formats under the Open Specification Promise (OSP) program. 

For information about the OSP program, visit this web site:
Microsoft Open Specification Promise

For download of the Office 2007 OOXML Specifications, visit:
Ecma-376: Office Open XML File Formats
Open XML Developer Site

For download of the Office 97-2003 BIFF Specifications, visit:
Microsoft Office Binary (doc, xls, ppt) File Formats