Is there an Excel viewer for Mac OS X?

Though not perfect: in Finder simply press the Spacebar to activate Quick Look, which offers a read-only view of many file formats without requiring to have the file's application installed.

You can use the Office Web Apps instead, which also allows you to edit files. You just need a Microsoft Account (you can use any email address to make one, I think). Just pop the Excel file in your OneDrive space and open it from there.

Here's an example file that I have publicly shared.

Numbers (from iWork) s capable of opening .xls and .xlsx files.