This howto is for those of you who are new KDE users, and also maybe for those long time users with bad memory.

Why bother with shortcut keys?
There are two reasons why you should learn these shortcuts: Shortcuts are generally faster than using the mouse, so you can work faster. The second reason is that some shortcuts can open up new and useful functionality.
Global KDE shortcuts:
Open menu: Windows key
Open run command prompt: Alt + F2
Cycle through open windows: Alt + Tab
Browse through virtual desktops: Ctrl + F1, F2, F3, F4
Open desktop cube (if enabled): Ctrl + F11
Turn off desktop effects: Alt + Shift + F12
Unresponsive window killer: Ctrl + Alt + Esc
Minimize all windows: Ctrl + F12
Show all virtual desktops: Ctrl + F8
Show system monitor: Crtl + Esc
Next keyboard layout: Ctrl + Alt + K
Lock screen: Ctrl + Alt + L
Dolphin file manager shortcut keys:
Rename file: F2
Split screen: F3
Open console: F4
Refresh: F5
Editable address: F6
List folders: F7
Make new folder: F10
Send file to wastebin: Del
Permanently delete file: Shift + Del
Show filter line: Ctrl + I
Increase size: Ctrl+
Decrease size: Ctrl-
Important: All of these (and more) are found under Systemsettings --> Shortcuts. If you would like to edit the shortcut keys, it can be done there.
Edit: Added a few images.
The desktop cube (Ctrl + F11):

Showing all virtual desktops (Ctrl + F8):

Cycling through open windows (Alt + Tab):

Using Dolphin file manager:
