LetterShifter
This is a program that shifts the each letter you give in a message down the alphabet by a specified amount. For example, the string "Test" shifted down 2 would give back the message "Vguv".
It's a rudimentary way of encoding data or messages so that they're not immediately obvious as to what they are.
This is one of the first programs that I learned how to create, back before high school, but I lost my original source code (that one was written in C++!), so I quickly recreated it in C# 'cause I wanted to use it again for something I was writing.