Lucidchart is a collaborative chart creation tool that I reviewed a few years ago. Since then Lucidchart has continued to update with new features.

Lucidchart is an easy tool to use to create flowcharts, mindmaps, and graphic organizers. Lucidchart offers a simple
drag and drop interface for creating flow charts, organizational charts,
mind maps, and other types of diagrams. To create with Lucidchart just
select elements from the menus and drag them to the canvas. You can
re-size any element and type text within elements on your chart. Arrows
and connecting lines can be re-sized, repositioned, and labeled to bring
clarity to your diagrams.

Lucidchart now offers Google Drive integration through a Chrome web app. If your school uses Google Apps for Education, you can add Lucidchart to your domain. Both of these options make it easy for students to save and share their work online.

Lucidchart charges business customers, but makes all of their tools free for teachers and students.

Applications for Education
Watch the video below for an example of how to create a Lucidchart graphic organizer for students to use. Click here if you cannot see the video.