For users that only occasionally need to open some Excel document there are a lot of free alternatives like Bytescout XLS Viewer.
Bytescout XLS Viewer is, as its name indicates, and XLS file viewer compatible with Microsoft Office (.XLS and .XLSX) and OpenOffice (.ODS) that functions without the need of installing one of the mentioned programs. It serves you perfectly to open or edit Excel documents if necessary.
The editing options are minimal, although if you don't have any office software installed then possibly you won't need anything more. Bytescout XLS Viewer takes care of the sheets, columns and cells created in each document, and offers a very organized view of the content that, in addition, can be exported to HTML.