Power adapter blocking adjacent plugs
Some wall-warts are made to plug in horizontally to avoid that problem. But then some power strips are arranged horizontally also, so it is blocked one way or the other. The better wall-warts are made to extend OUT from the plug rather than below or to the side of the plug. That way they don't interfere with adjacent plugs, but then they are too long to plug into the wall behind a piece of furniture, etc.  There is no universal solution to this problem.

Except maybe the really short (1 ft/ 30cm) "extension cords". They are made speciically to solve the problem that you present.  Here is one example of many available...


http://www.allelectronics.com/item/ext-11/1-ft-extension-cord/1.html