Oh! Really good and interesting thread!
What about me, my screen name is rather old, I 'invented' it about 3 years ago. I was doing my English homework and I was looking through my dictionary. And when I saw the word "magnet" (seriously I wasn't looking exactly for it), I suddenly thought of the word "magness". Later I decided to use it as a nickname on one site. And then it became my screen name almost everywhere, for some reason I liked it...