Microsoft Excel Viewer is a simple application which allows you to open Excel sheets without Excel. Once installed, just double-click on the file you want to open and Microsoft Excel Viewer does the rest:
It's just like using the real thing but obviously, there's no editing of any kind allowed but it's ideal for just browsing or viewing Excel documents if you don't have Office or your Excel isn't working.
If you need to edit Excel, you can opt for OpenOffice which is completely free to use and allows you to both open and edit Excel, PowerPoint, Word and many other formats although it's much heavier and requires a longer installation process than Microsoft Excel Viewer. There's also LibreOffice which you can read more about here.
Download Microsoft Excel Viewer.