With the shortcut alt+F2 in KDE you invoke the "run command" dialog (also accessible from the start menu if it's not hidden of course). It is a fast and easy way to launch an application if you know its command line name (which is mostly the same as its normal name, except for things like OpenOffice.org).
Beside this mini command line functionality it also works as a simple calculator, which can be very handy. It is limited to integer calculations, but beside the classical arithmetic operations (+, -, *, /, ()), it is also possible to do bit shifting (<<, >>) and logical operations(<, >, |, &).