OpenDocument Format
From LinuxReviews
The OpenDocument Format (ODF) is the file-format used to save documents by OpenOffice, StarOffice, and other programs. Many free software programs like KOffice know how to load and save this format. The file sare saved with the extention .odf.
[edit] Windows users
SUN has releasted a plugin for MS Word which allows it to load and save OpenDocument files[1]