XLSX
Office Open XML Spreadsheet (.XLSX) is the default file format for documents used by Microsoft Excel as of Excel 2007. In prior versions, the default version was XLS.
History
This (along with the other Office Open XML items DOCX and PPTX) was initially standardized as ECMA-376 in 2006. Three formats of this standard have been produced; the second version also corresponds to ISO/IEC 29500.
Incompatibility with earlier versions
Attempting to open XLSX files with earlier versions of Excel (pre-2007) results in garbage instead of a proper spreadsheet. A compatibility pack supposedly adds the ability to load the newer format into the older versions, but this doesn't always seem to work. This can be a problem when people insist on e-mailing you files in the newest proprietary Microsoft formats when, most of the time, whatever they're sending could have been done fine in an entirely nonproprietary keep-it-simple-stupid format such as CSV or plain text. Open Office can open XLSX files, however.
