Gliffy

Gliffy is an online Diagram creation website that would be a great way for students to create flow charts.  What is great is that no sign up is required and it is free to use.  When you are creating a new document, you choose the format (category) that you would like for your diagram.  It even has the ability to create floor plans/Interior Design.  (I used to have my students create their dream house when doing the French Revolution and discussing the Palace of Versailles, this would work great).  The site has business in mind for the majority of its purpose, but it could still easily be used in the school setting. There is the ability to collaborate on a diagram as well as share it on the Internet. 

To get more features though you have to sign up for a premium account.  This is available for a 30 day trial.  After that, you get a to choose between upgrading to a premium account or keeping the free basic account.  This allows 2MB image upload limit and the creation of 5 public diagrams.  For more information, you can view their product comparison to see if this is something you might be interested in using with your students.