I very much like Google Draw. It has the benefits that I love about Google Docs in general. Using it with students is great because it is a simple program to not only let them easily create graphics for so many purposes, but can be shared with the teacher and other students for collaboration.
A staple of graphic organizers is the Venn Diagram. For that you really want to be able to change the opacity (translucency) of graphics so you can see the intersection. This is also important for making infographics.