Simple Diagrams offers a free tool that anyone can use to create diagrams using a combination of clip art, text, and free hand drawings. Simple Diagrams provides a large selection of shapes and drawings that you can drag and drop into your diagrams. You can adjust the size of each element you place in your diagram. Any element can also be altered by using the pencil drawing tool. The pencil tool can also be used to create a drawing from scratch. View my diagram below (click to enlarge).
To use Simple Diagrams you do need to install the Simple Diagrams software. Simple Diagrams uses the Adobe Air platform which means you can install Simple Diagrams on both Mac and Windows computers. Simple Diagrams offers a free version and a paid version. The free version appears to offer plenty of tools for classroom use.
Applications for Education
Simple Diagrams could be a great tool for creating flow charts and sketches. Using some of the ideas I've discovered in Unfolding the Napkin, students could use Simple Diagrams to create images depicting a problem and their potential solutions to a problem.
Here are some related items that may be of interest to you:
Periodic Table of Visualization Methods
Nine Tools for Collaboratively Creating Mind Maps