Mine was quite easy really. Back in circa 1997 when I first used t'internet I found Stephen was taken and tried a few things out that I didn't like. Couple of plays on my surname, few other things but nothing stuck.
Then I remembered watching a film called House Guest and there was a character called "ST-3". So I changed the letter E for the number 3 like any good 31337 person would do and kept using it.
Remember though, Stephen with a capital S, st3ph3n with a small s. It looks cooler that way [H]