One of the key planks of Microsoft's appeal to businesses is the ubiquity of Microsoft Office. This in turn has been built on the strength of the file formats used by Word, Excel, and PowerPoint. Microsoft originally released the OpenXML SDK back in 2007 but has now released a new version (OpenXML SDK version 2.5) under the Apache 2.0 license. The change in license terms makes the use of the technology much more palatable to those wanting to use the technology in their own projects.
This spirit of openness applies to both the SDK itself as well as the documentation surrounding the project and the file formats. They join the recently released OpenXML Package Editor for Visual Studio 2012 and 2013 which is also now an open source project. Microsoft’s Brian Jones observes that the according to Microsoft data, the three biggest uses of the SDK are for document generation, content manipulation, and data import/export.
For those looking to see the code in action, Eric White has prepared a short YouTube clip and a blog post that provides a short introduction into using the SDK. White also observes that there is no practical or technical reason for the SDK to be Microsoft only, and he plans to ports via Mono to OS X and Linux.