title: Best WordPress plugins to learn how to build WordPress plugins
body: <p>Not sure this question is appropriate for stack overflow
  but on the other hand I'm not quite sure where else to
  ask it.</p>
  
  <p>I am learning about about WordPress plugins and would like
  to ask for recommendations as to which WordPress plugins to
  download and analyse so as to learn by inspecting their
  source code. What are some of the best ones that would
  do for instructional purposes?</p>
  
  <p>Thanks.</p>