Microsoft Excel Viewer is a free tool with which “you can see, open and print all Excel files, even when Excel is not installed. You can even copy everything you have in Excel to other programs".
However, Microsoft Excel Viewer “will not allow you to edit or modify" the documents you open. You cannot save books or create new ones either. For that purpose, you will need to have Excel installed.
Microsoft Excel Viewer helps use all documents generated in Excel that you cannot use due to not having this application, thus becoming in a necessary tool for any user.