Open Excel XLS XLSX spreadsheets and CSV, ODS files with Bytescout XLS Viewer
Bytescout XLS Viewer is a free (for personal use) program that lets you view XLS, XLSX, CSV and ODS files without using Microsoft Office. The interface is not particularly elegant, but it is convenient enough.
In case you are going to be using it often as a viewer for these file types, the program can be associated with such files.
Note that some very basic operations are also possible, such as copy and paste cells or print. You can also export a file in HTML format ready to be published on-line.