JGraph is a UK company that develops and supports graph visualization software and web services. One of the free services they offer is a diagram creation tool called Diagram.ly. Diagram.ly offers a drag and drop interface for creating diagrams using clip art and pre-drawn shapes. Using the service does not require registration and all of your diagrams can be saved to your local computer in your choice of four formats (xml, png, jpg, or svg).
Applications for Education
Diagram.ly could be a good tool for students to use to create flowcharts of a process or concept. Students could also use Diagram.ly to create mindmaps that use images instead of just words and lines. As registration is not required in order to use the service you can have students using it quickly without losing instructional time to walking students through a registration process.