Franklin Veaux created this giant, very detailed map of all the different kinds of non-monogamous relationships you can have and how the different "kinds" (swinging, cheating, polyamory, etc.) overlap with one another. Believe me when I say that you could spend a long time studying this thing and still not have noticed everything:
Of course, you probably can't read that unless you have the eyes of a bird of prey, so you can head over to Veaux's journal for a much larger view.
Or, you could play this fun game: go back into Nerve's "I Did It For Science" archives and see how many of the categories up there you can find as an article! Winner gets the satisfaction of browsing through some very entertaining reads.