What was a useful tool before is now even more useful.  Lucidchart is a concept map, venn diagram, chart creating tool that works very similarly to a Google Doc and can be connected to your Google Drive.  There is a collaborative feature to Lucidchart where multiple students, or teachers, could be working on the same chart at the same time.  The Add-on allows you to more easily place your charts into your Google Docs for sharing and completing assignments.  You can quickly create a chart or diagram to help explain a concept or idea and then place that directly into your Doc.