My favorite free tool is Google Drawings, it is fantastic for graphic design. The web-based app is user friendly and allows for infinite possibilities. When using drawings, having a rubric for required elements is key for students. (see below) Teachers can use Drawings to create activities and graphic organizers. (see below)
I use Drawings each week to create my Tech Tips 411 e-newsletter as well as other images. You are able to publish drawings to the web, export as PDF, jpg, png, or svg file formats.
Student Created Posters (6th grade Science)
Teacher Created Drawings
Check out some teacher creations from a professional development session I provided on Google Drawings.