I don’t know if those shortcuts are available everywhere Compiz is used or if they are exclusive to KDE, but:
- CTRL + F12: puts all windows on the background and displays a ghost window with your widgets. It is the “Plasma Panel”
- CTRL + F10 (also accessible moving the mouse pointer to the upper left corner): shows all windows opened and make them clickable so that you can change to that application. If there is something dynamic happening — like a video being played — then you can see it being updated in real time.
- CTRL + F9: zooms out so that you can see all applications from that particular desktop, side by side, and click on them to choose which one you want to be active.
- CTRL + F8: zooms out and shows you the current view for all existing virtual desktops.
The ESC key exits from all of those views.
Using them makes it better to see what is happening on multiple environments.