A more lowbrow example:
The webcomic "Order of the Stick". for a while, had a character who was unable to speak intelligibly as the result of a curse. This character's speech bubbles were full of meaningless strings of letters. However, as I read it it became pretty clear that there was some kind of substitution going on. It was a simple substitution cipher, so it was possible with some effort to make sense of this character's dialogue. The cipher would change every few strips, and I'm not aware of any clue or key given within the comic itself.
Figuring out how to read this had a little to do with knowing the typical frequency of letters in English, but was probably more down to context (predicting what the character might say) and the fact that the length and capitalization of the words remained intact.
I just checked an old episode there, and was very quickly able to decipher the unintelligible dialogue, but this was almost entirely down to knowing the context. I remember when I initially read it, I more or less memorized the ciphers without specific effort.
Predicting what a rot13ed text might say could be a factor, and in my experience it was not particularly difficult to assimilate similar ciphers once I was reasonably confident my decryptions were correct.
rot13.com is a service frequently used here to hide spoilers. I really hate it, though. If I had the time, I would build a simple, but much better alternative. Maybe somebody has more time to do that, so I'll share my rough specification:
The only important design problem I don't know how best to solve is making the encryption work for Unicode, with the following three constraints: making it a reciprocal cypher, outputting visually nice strings, and making it map ASCII to ASCII. One possible solution is to drop the constraint that it is a reciprocal cypher. For this service it is probably not crucial anyway: the ciphertext can be base64, with some escape prefix distinguishing it from plaintext.
After writing the above, I found this LW thread: Does anyone else find ROT13 spoilers as annoying as I do? There were several suggestions there, and two of the commenters, sketerpot and LightningRose even coded their own solutions to the spoiler problem. Each solution had some merit, and LightningRose's in particular was far superior to the rot13.com site I used to use, but basically, they only dealt with the third point of my proposal.
Is there anything like what I envision? Is anyone interested in building it? What changes or extra features would you like to see?