Way back in the olden days of the internets, I somehow came up with "DaFace" as a username to use in the chat forums and the like. I was stupid and in high school and thought it was also cool to have an overgrown smilie that was the visual representation of DaFace:
Clever, I know. Anyway, when I started getting into forums that used avatars, I needed a similar representation since the text gizmo didn't seem all that great. So I started looking around online for faces, and came up with the Hitchhiker's Guide face as a good option:
Over the years, I got tired of that one, but I hate having trouble figuring out who people are when they change avatars (or around here, usernames), so I've always stuck with the theme of some sort of green face. I probably change it once a year or two, but it's always a green face.