Most of my friends know me as skippy than know me as Robert. The original reason for it was the fact that everybody knows somebody named Robert (I have a whole rant I once wrote about Robert-Americans and how we are large enough minority to demand special treatment, if we would only vote as a block), so, using the old joke about the evil twin, I became "skippy Rob".
As a dealer at cons, I found that people could remember not only me, but also the name skippy Rob, and it made for a great introductions: "Hi, I'm skippy Rob. I am my own evil twin...saves on clothing costs." If I got a giggle or a smile from this bit of sillieness, I usually made a sale also.
In my SCA household, there was another Robert, who became the Good Twin, and I mostly dropped the Rob, becoming skippy the Evil Twin. To further distinguish myself from any other Skippys (such as a TV character), I dropped the cap 'S', joking that I couldn't afford the Capital Gains taxes.
Since I am pagan, proud of this fact, and very much out of the broom closet, when I first got an email address, I realized that skippy_the_witch was easy to remember for both me and others. Thus, my screen name.
skippy
The GM of the Cursed Earth Campaign