The launcher built into the KDE Plasma desktop. While its basic function is to launch programs from a sort of mini-command-line, its functionality can be extended by "runners" to assist the user to accomplish a lot of tasks.
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Pro Can be used to open websites
By just typing an URL into the text box users can have the site open in their default launcher.
Pro Opens folders/files also
Just by typing name of file or folder, it will give recommendations
As it is written in Qt QtQuick using QML.
Pro Extensible with plugins
Krunner can be extended with the use of "runners" which are essentially plugins for the app launcher to allow for extra functions.
Pro Lots of handy features.
Such as a fully featured calculator, file opening, browser functions, currency conversion, date/time functions, etc.