Good thread idea.
Smogon was (and still is) the only forum I had really joined when I signed up for smogon. I hadn't really lurked in the forums section (mistake), so I had no precedent or pre-existing thought on usernames. So I decided to go with Emerald, what I choose to call the protagonist of Pokemon Emerald (the names based on game title were way better imo). The only problem was that the name Emerald was already registered on shoddy, and I wanted my usernames to match, so Emerald became 3merald. After a little while, the 1337 really started bugging me, so I wanted to switch again. I figured if I was Emerald from the game I had beaten, that made me King Emerald right? King Emerald stuck for a while, but I did some stupid things here (compared to some stuff that happens here, not that stupid, but I'm a perfectionist and can't stand messing up) and when I came back after a few months of hiatus, I wanted to show the change. On irc, my nick was KingE, and there was a brief conversation about me being the 'upgrade' of Mr. E, and also an inquiry about Ace E. So I figured I should level up from King to Ace, and I'm pretty happy with it. Not that it matters, cause I'm out of name changes.