XRechnung Viewer
The XRechnung format is a E-Government standard for electronic invoicing. At some point it will be mandatory for every company dealing with German governmental partners to send the invoices in this XML format.
Many commercial vendors have already caught up and provide ways to generate XRechnung formatted documents with their software. However, to my knowledge, the availability of open source end user software is very limited. Since the standard itself is at least very open and transparently documented, so I think it is worthwhile to also support it with free software on the desktop.