With a browser, right-clicking will allow you to save it as wallpaper, open a link in a new window, or copy it on to your clipboard.

 Pressing Alt-F4 will usually allow you to quit.

If you want to minimize everything or something that doesn't usually minimize, right-click the bar on the bottom and choose "Minimize All Windows".

Press Alt-Tab to switch between two or more programs. Release alt to choose that program. Works in full screen programs too!

 If all else fails, press Ctrl-Alt-Delete (not backspace) to bring up the close program menu.

 If it won't appear because there is a very big problem that can't be switched out of, press Ctrl-Alt-Delete some more to restart the computer.