
KDE Platform 4 was a collection of libraries and software frameworks by KDE that served as technological foundation for KDE Software Compilation 4 distributed under the GNU Lesser General Public License (LGPL). KDE Platform 4 was the successor to KDElibs and the predecessor of KDE Frameworks. KDE Platform 4 is the only version of KDE Platform, see KDE’s brand repositioning.

Akonadi is a storage service for personal information management (PIM) data and metadata named after the oracle goddess of justice in Ghana. It is one of the “pillars” behind the KDE SC 4 project, although it is designed to be used in any desktop environment. It is extensible and provides concurrent read, write, and query access.

KHTML is a browser engine developed by the KDE project. It is the engine used by the Konqueror web browser. Although it has not seen significant development since 2016, it is still actively maintained, and engines descended from KHTML are used by some of the world's most widely used browsers, among them Google Chrome, Safari, Opera, Vivaldi, and Microsoft Edge. Distributed under the terms of the GNU Lesser General Public License, KHTML is free software.

KIO is a system library incorporated into KDE Frameworks 5 and KDE Software Compilation 4. It provides access to files, web sites and other resources through a single consistent API. Applications, such as Konqueror and Dolphin, which are written using this framework, can operate on files stored on remote servers in exactly the same way as they operate on those stored locally, effectively making KDE network-transparent. This allows for an application like Konqueror to be both a file manager as well as a web browser.

The Oxygen Project is a project created to give a visual refresh to KDE Plasma Workspaces.

Phonon is the multimedia API provided by KDE and is the standard abstraction for handling multimedia streams within KDE software and also used by several Qt applications.

ThreadWeaver is a system library initially developed for KDE Software Compilation 4 and later refactored for KDE Frameworks 5.