Another nice type of 'folk theorems' I have seen is of a sort where some relatively straight forward generalization of a well established theorem is assumed and then used for its heuristic or explanatory value. I find this is often used in fields where mathematicians interact with non-mathematicians and although it is completely non-rigorous (and sometimes even misleading!) most often it helps in exposition and for building intuition.
An example would be the "folk theorem of evolutionary game theory" (as used by Hofbauer and Sigmund, BAMS 2003) on certain kinds of correspondences of Nash equilibrium and dynamic approaches.
