Rot13 is both an encoder and decoder for languages that use 26 letters in their alphabet. You can enter plain text or encoded text, and you will be given the other one. Just type either one here and it will be automatically encoded or decoded. This implementaion will work with other alphabet lengths as long as they contain an even number of letters. Otherwise, you will only see a warning and it won't halfway work.
I also made a "rotN" encoder, also called a Caesarian Shift. This will let you pick any number besides 13, and is a much faster way to cycle through different possibilities. Also, the Vigenère cipher is somewhat similar.