Mine is the first two letters of my first name Terje, and the first two letters of my last name. Of course, the first two letters of my last name are Hø, but I first came up with it when registering at GameFAQs, and they wouldn't allow non-english characters in usernames back then.

I also used it for Classicamiga because when we first created the forum everybody knew me as Teho from GameFAQs, so it made sense to keep using it. And still does. But for many years now I've just used my real first name as my usernames when I register at places. I sometimes use Teho when Terje is taken by someone else, but that rarely happens.