Just saw this over on Brian Jones' Blog, and I'm surprised this hasn't made more of a splash on the interwebs... Microsoft has just released the specification documentation for our binary Word, Excel, and PowerPoint file formats. This means that if you write another office program like OpenOffice, Lotus Symphony, StarOffice, etc., you can ensure that your app can read and write the Microsoft Office Binary file formats exactly as they were intended.
The binary documentation itself is available up here: http://www.microsoft.com/interop/docs/OfficeBinaryFormats.mspx
It's all covered under the Open Specification Promise.
As I mentioned earlier , Microsoft has released the specifications for our binary Office 97-2003 file