A ROT Cipher Encoder/Decoder available here.
How does it work?
rotN simply rotates characters of a given text by N positions. ROT13 is a special case, since applying the algorithm twice would yield the original text. You can find more information about it here.
Is it useful?
Although very insecure, the ROT series of ciphers are great at masking text. An example would be hiding spoiler text or pretending to be cool at school.
Dude 1> Hey man, how's it going? Dude 2> Nyy tbbq bire urer va gur EBG havirefr! Dude 1> Huh?