You can also open and edit XLSX files without Excel, completely free, using WPS Office Spreadsheets, OpenOffice Calc, or LibreOffice Calc. If you're on a Mac, Apple Numbers supports XLSX files ...

Designed for use with Office 2000, XP, and 2003, the downloadable add-on allows you to open, edit, and save the newer file formats, seamlessly converting the files on the fly and in the background.