dcop

dcop is KDE's access-to-everything tool, which most KDE apps export some sort of interface to. For example, to ask the KWeatherService the current temperature in Canberra (which has station code 'YSCB'):

[jk@b4 jk]$ dcop KWeatherService WeatherService temperature YSCB
12°C

or to get the currently playing track from noatun:

[jk@b4 jk]$ dcop noatun-8897 Noatun title
[The Aphex Twin] - Polynomial-C

or make kmail check for new mail:

[jk@b4 jk]$ dcop kontact KMailIface checkMail

Handy for scripting!