In computer software a widget engine is a software service available to users for running and displaying applets on a graphical user interface such as that of the desktop.

The term widget engine is not to be confused with that of a widget toolkit. Toolkits are destined to GUI programmers who combine several widgets to form a single application. A widget in a toolkit provide a single low level interaction and is prepared to communicate with other widgets in the toolkit. On the other hand widget engines are intended for end users and each desktop widget while serving only a purpose that is usually addressed by the effort of one GUI widget in a full-scale application is itself a stand-alone task-oriented application which can be composed of several related interactions on its own.

The desktop widget model is attractive because of ease of development. Most of these widgets can be created with a few images and about 10 to several hundred lines of XML/ JavaScript/ VBScript source code. A single host software system such as a web browser runs all the loaded widgets. This allows several desktop widgets to be built sharing resources and code.