My favorite as of this writing is the elegant Coggle. Coggle creates visually beautiful org charts. It only does one thing, but in a remarkably straightforward way. The organizers it creates all more or less will look the same. Yet, its ease of use and graceful layout make it a powerful tool for the classroom.
Family trees, language derivatives, essay mapping, biology nomenclature hierarchies, history timelines immediately spring to mind as ways a teacher could use this tool.
On the tech side, Coggle lets one embed links and pictures. It allows for synchronous and asynchronous collaboration. One can also share a finished Coggle with others both privately and publicly. Students from elementary grades through college age will find this a useful tool.