I've been exploring several drawing tools, particularly those for making graphs and diagrams. I'm trying to stick to free and online in this discussion, though a few for-pay tools have some nice advantages, mainly storage space and collaboration.
Among other advantages, drawing tools can help students mind-map and brainstorm; collect and display numerical data in charts and graphs; demonstrate reflective learning in storyboards or networked images; and so on. Graphing skills become increasingly important as an academic tool as students progress through school, but charts and graphs can be a fun motivation even for younger students.
Gliffy is one of my old favorites, but it limits you to just 5 drawings, unless you go for the somewhat pricey paid account (5 users for up to 200 drawings, for about $10, as of this writing). It does very nice Venn diagrams from templates, has loads of pre-formed objects, such as arrows and rectangles, and supports HTML5.
wire frames, mind maps, network charts, and site maps . . . simply pick and "drag and drop" elements from a large library of stencils.Cacoo is one of the free programs with a free-hand drawing option, too.
Creately is another free program with great features, and like Gliffy, allows up to 5 drawings with limited collaborative possibilities. It gives you only diagrams, but offers nice Venn templates, and a large selection of templates for K-12, including:
...Storyboards, Fishbone Diagrams, T Charts, Y Charts, Venn Diagrams, and much more..
Google Drawings has only a very basic toolkit, so don't expect a great deal, but it is quick and easy and the interface will be familiar from Google docs.
If you want a very professional look, but have only a limited project, try Microsoft's Visio or SmartDraw. Both of these have a free trial period and many features.