I am quite biased towards the diagramming tools and Gliffy seems to be a good place to start for the basics. As an educator or a student, the software is simple to use. It allows one to publish and/or share for collaboration. So it can a be a great tool for an educator to get students involved in a topic or subject. Students simply add to the diagram their input from wherever and whenever. The educator/administrator simply adds their email address as a collaborator.
Gliffy allows users to create several different types of diagrams. They include: network layouts, Venn diagrams, organization charts, flow charts, and SWOT charts (Strengths, Weaknesses, Opportunities, Threats). Users can insert images. (There is limited, but a fairly substantial amount of images within the site a user can utilize.) Simple, but very helpful feature is the undo. Not all diagramming sites allow this ability, so I found it quite gratifying that I was able to so as I was creating a simple diagram of our Federal government organization.
Gliffy is initially free, but for only the first 30 days once a user sets up an account. After that it costs $4.95 per month allowing users up to 200 diagrams, 200 Mb of storage, public and private diagrams, and unlimited collaborators. Another subscription rate is $9.95 per month. This includes unlimited diagrams, unlimited storage, public and private diagrams, and unlimited collaborators. For educational purposes the standard $4.95 per month should suffice for most educators. http://www.gliffy.com/publish/3515170/
Other useful attributes is it can be added to a Wordpress site as part of a user's blog or website. Users can add it to their Google Apps account for simple, no login entry required entrance. Gliffy diagrams can be added into Word documents, PowerPoint presentations, and Excel spreadsheets.