The Microsoft OOXML SDK allows for easy reading/writing/manipulating of Open XML documents like DOCX, XLSX and PPTX files. For finally benefiting open-source developers, the Open XML SDK was released today under the Apache 2.0 license.
These Open XML libraries can be found hosted on GitHub. More information via MSOpenTech.com.
This work may benefit some open-source document editors / office suite software, with more commentary being available from Michael Meeks' blog.