Is there any software, preferably free, preferably supporting drag & drop, preferably supporting save & load to ease the creation diagrams like these? 

<img src="https://www.sqlite.org/images/syntax/select-stmt.gif" width="300" />

<img src="http://v4manual.tikilive.org/wp-content/uploads/2011/07/connecting-video-source-1.png" width="300" />