ALT-F2 Magic
Most KDE users probably know what pressing ALT and F2 does, it's opening the "Run Command" dialog. But did you also know that this little dialog can do magic? Try "2+2 ", try "schumacher@kde.org", try "www.kde.org", try "help", try "logout", "gg:sin 5 ", "leo:warzenschwein" or "ggi:warthog".
The little program implementing this is called "minicli". It comes in various flavors. ALT-F2 is one of them, but you can also find it in the K-menu as "Run Command" enntry or add it to your desktop panel as "Application Launcher" applet. If you are interested in the source have a look at kdebase/kdesktop/minicli.cpp.
The surprising thing is that all this features are well-hidden. There don't seem to be many people knowing them. I also did not know about them before Waldo gave a hint and Danimo reemphasized it.
This all came in the context of the calculator fever on Planet KDE, a nice debate and programming happening, from what I learned some interesting facts about Superman.