Microsoft Excel™ 2003 shipped with a file format called SpreadsheetML. This file is XML. It is not very common, but its core features are supported.
| Name: | Excel |
| Extensions: | .xlsx |
| Compatible Programs: | Microsoft Excel, Microsoft Office, and many other spreadsheets |
| Open / Save: | Both |
Supported by the 'MS Excel (tm)' plugin.
These are zip files containing SpreadsheetML files and binary files.
This file format is not very widespread and therefore support by 3rd-party applications is limited.