Plasma replaces KDesktop in KDE4. Its implementation is based on the Qt Graphics View Framework, which was introduced in Qt 4.2. For more information about Plasma, refer to https://techbase.kde.org/Development/Architecture/KDE4/Plasma.

Sonnet is a multilingual spell-checking application that supports automatic language detection, primary/backup dictionaries, and other useful features. It replaces kspell2 in KDE4.

The KIO library provides a framework for network-transparent file handling, allowing users to easily access files through network-transparent protocols. It also helps provides standard file dialogs.

KJS and KHTML are fully-fledged JavaScript and HTML engines used by different applications native to KDE4 (such as konqueror).

Solid is a hardware and network awareness framework that allows you to develop applications with hardware interaction features. Its comprehensive API provides the necessary abstraction to support cross-platform application development. For more information, refer to https://techbase.kde.org/Development/Architecture/KDE4/Solid.

Phonon is a multimedia framework that helps you develop applications with multimedia functionalities. It facilitates the usage of media capabilities within KDE. For more information, refer to https://techbase.kde.org/Development/Architecture/KDE4/Phonon.

Telepathy provides a real-time communication and collaboration framework within KDE4. Its primary function is to tighten integration between different components within KDE. For a brief overview on the project, refer to https://community.kde.org/Real-Time_Communication_and_Collaboration.

Akonadi provides a framework for centralizing storage of Parallel Infrastructure Management (PIM) components. For more information, refer to https://techbase.kde.org/Development/Architecture/KDE4/Akonadi.

KDE4 also features an easy-to-use Qt-based framework for adding online help capabilities to applications. Such capabilities include tooltips, hover-help information, and khelpcenter manuals. For a brief overview on online help within KDE4, refer to https://techbase.kde.org/Development/Architecture/KDE4/Providing_Online_Help.

KXMLGUI is a framework for designing user interfaces using XML. This framework allows you to design UI elements based on "actions" (defined by the developer) without having to revise source code. For more information, refer to https://developer.kde.org/documentation/library/kdeqt/kde3arch/xmlgui.html.

Strigi is a desktop search daemon compatible with many desktop environments and operating systems. It uses its own jstream system which allows for deep indexing of files. For more information on the development of Strigi, refer to https://www.vandenoever.info/software/strigi/.

KNewStuff2 is a collaborative data sharing library used by many KDE4 applications. For more information, refer to https://techbase.kde.org/Projects/KNS2.