You could use Google Docs if you wanted to use the cloud.
It's supported xlsx since 2009
A new version of Google Spreadsheets is here! Make sure to check out the following features:
XLSX import. Excel '07 files (xlsx) can now be successfully imported into Google spreadsheets, supporting cell data, formatting, formulas, font style, background color, named ranges, frozen panes, and horizontal merges. Note: there's currently no option to export files as .xlsx.
Just note that last line about exporting them though.
Important edit
I've just checked, exporting as ".xlsx" is now supported via the "Download as" button