BoingBoing recently linked to a flowchart comparing how the scientific method and religious faith build knowledge. The contrast between science and religion is great, but could be somewhat offensive to religiously inclined students1. However, the scientific method flowchart is clear and informative by itself; I think it would be a great aid for explaining the scientific method to introductory students.
The image is by Wellington Grey, and is Creative Commons licensed.
[Update just before posting: PZ Myers has posted the images as separate figures; his versions should be easier to use in class.]
1 I'm torn about using the faith portion of this in my classes; on the one hand it is a decently accurate representation of how faith-based systems build knowledge, but on the other hand religion is such a taboo topic that it seems easier to leave that part of the figure out.