If it exists, there is porn of it -- no exceptions.
Supposedly originating and popularized by the 4chan Image Boards, this was the first near-universally agreed-upon Rule Of The Internet. It is so well founded and documented with irrefutable proof that even those with only a cursory awareness of the Internet are aware of this rule even if they don't know it has a name.
While the original architects of the Internet had grandiose goals of research and data sharing, the second it fell into the hands of Joe Everyman, it became a tool for one thing and one thing only: pornography! Now, it's not that everyone online is just looking for pornography; it's just that it's very very easy to come across. Even if you're not looking for it! Don't believe us? Do a Google image search (filters off) of, well, pretty much anything. Sometimes even with the filters on.
The key reason the scope of it is so wide and bizarre lies in what some have come to call Rule 36: "If you've thought of it, then there's somebody out there with a Fetish for it." (And incidentally, by "it" we mean "anything that exists in the world.")
There's also Rule 35, basically a guarantee that Rule 34 will remain true: "If there is no porn of it, it will be made". This basically means that if you notice you can't find porn of something, and point it out, somebody will be happy to draw/write/find it for you in pretty short order.
Finally, there is the concept of quantum porn. "Referring to a type of previously non-existent porn will cause online porn of that type to come into being retroactively." Nobody takes this seriously, so far as is known.
You may wish to keep a bottle of Brain Bleach handy while proving Rule 34. See also Rule 63, which gets mixed up with this. And if you do go hunting to prove this rule false, say good-bye to your childhood first...
Not to be confused with US federal courts' Rule 34. Or Rule 34 of the Evil Overlord List (dealing with the trope Scaled Up). Or The 34th Rule, a Star Trek: Deep Space Nine book (the 34th Rule of Acquisition reads "War is good for business"). Or Wolfram's Rule 34. Or Charles Stross's novel Rule 34 (although it is the origin of the Stross title).
Nobody's sure if it's a coincidence that Lyons Township High School's rule book has #34: No Pornography; mostly because nobody's ever dared to ask.
No examples, please. This applies to everything (except for Discworld, illustrated on several websites, as well as fellow novel runs Leviathan and Mortal Engines), so the list would be as long as ... everything. Also, we're not really interested in being the Net's "How To Find Rule 34 Stuff"—you will need to look elsewhere if you want to find that kind of thing.
For a list of creator reactions to the phenomenon that are definitely not examples, see here.
- Except for Simtunes.