The OpenDocument format (ODF) is a free and open file format for electronic office documents, such as spreadsheets, presentations and word processing documents. ODF if primarily implemented by OpenOffice.org office suite and the main file format of the respective product.
This file format was popular among linux (Primarily open source developers as well) fans but the pain was using these documents under a Windows PC having MS Office. MS Office does not natively support OpenDocument currently. However you can download ODF add in for Microsoft Office at http://sourceforge.net/projects/odf-converter. And the MS Office will natively support the ODF format from SP2 of Office 2007 onwards.