I just learned about a free web-based tool called 'Crappy Graphs' (http://crappygraphs.com/user_graphs/makecrap.php) that if you're not trying to make a crappy graph, would actually be quite useful to quickly make a Line Graph or a Venn Diagram to illustrate concepts in logic, set theory, etc. I've included a few examples of what it can do. See the full gallery on Posterous After note: You may want to check out http://grapholite.com/ as well.
A student in one of my classes a few years ago did a project on The Number System. This was the result. It gave me a lot to think about in terms of just how to present Venn Diagrams. See the full gallery on Posterous