For obvious reasons, Microsoft is not very keen on adding native Open Document Format support in Microsoft Office so rival Sun released a free ODF Plug-in that makes your Office programs ODF friendly.
Using the ODF Plug-in from Sun, Microsoft Office users can create, read or save documents, spreadsheets and presentations in Open Document Format (ODF) using the stand Microsoft Office programs.
The plug-in therefore offers two-way conversion of Microsoft Office documents to and from Open Document. Unfortunately, it doesn't support Office 2007 yet though the previous versions of Office are well-supported.
Earlier, a similar plug-in was tested by the ODF Foundation. Microsoft is pushing it's own Open XML document format but a Garner report find it unlikely that ISO will ever adopt Microsoft's Open XML document format.
Here's how to use the ODF format in various Office programs:
In Word, choose File - Save As and select the file format "ODF Text Document (*.odt)"
In Excel, choose File - Export ODF Spreadsheet.
In PowerPoint, choose File - Export ODF Presentation
Related: Microsoft XPS vs Adobe PDF