Unlike Firefox which needs an extension (as mentioned by ThomasZ just now), KDE’s Konqueror users can happily and easily view OpenDocument files thanks to the integration between KOffice and the rest of KDE. A simple click on the ODF file is all that is needed.
Left side: Browse the document folder with Konqueror, and the click on the file.
Right side: OO.o Calc is used to create the test file.