In Compositing nodes window there is an option to quickly connect the selected node to a viewer node using some kind of keyboard shortcut. I can't remember what it was. The only thing I remember is, that you have to hover the cursor over the active node's output (as shown below), press RMB or LMB and some kind of shortcut. Thanks for help in advance.
One other thing is interesting to me. When I hover the cursor over the selected node's output and press SHIFT+Right-click it become highlighted. What is this kind of selection for?