OCS-API

OCS (OpenCollaborationServices) is a libre freedesktop standard used by pling.com and its various network sites. Read more information here:
https://www.freedesktop.org/wiki/Specifications/open-collaboration-services/
https://en.wikipedia.org/wiki/Open_Collaboration_Services

OCS-API in use

The OCS API is in use by various software projects, with the most prominent being KDE software, but also VLC. Apps use it for adding new content or addons like Falkon Browser or KPat or to theme the desktop via Discover or GHNS.
ocs-url: https://www.opendesktop.org/p/1136805/
KDE Attica Library: https://techbase.kde.org/Development/Tutorials/Collaboration/Attica/Introduction
KDE KNewStuff Frontend: https://techbase.kde.org/Development/Tutorials/Collaboration/HotNewStuff
NX Software Center: https://github.com/Nitrux/nx-software-center