You might know that Okular has a plugin system, for adding support for more document formats. And you might know that Calligra since years also provides a plugin to Okular, which adds support to view slides from files in the OpenDocument Presentation (ODP) format. And not only for the ODP format: by simply using the Calligra import filters for PPT and PPTX you can also view the slides locked away in those formats.
The different apps of Calligra used to be built on the KParts system, so any files in formats supported by them would be also viewable in KPart-embedding programs like Konqueror or KDevelop. But due to the currently on-going creation of a new MVC-oriented foundation for the Calligra programs this has changed, the Calligra modules are no more KParts.