Pizzini Cipher
The Pizzini cipher is very similar to the classic Caesar cipher where the alphabet is shifted three letters (A => D, B => E, etc). The difference is the Pizzini cipher assigns those letters to numbers so A = 4, etc. This cipher originated with Italian mob boss Bernardo Provenzano and is named after the little pieces of paper they used. Read more about it at Wikipedia.

Note: This cipher uses the Latin alphabet of 26 characters. The original version used the Italian alphabet, which only has 21 characters.