About 10 years ago, Leanne and Megan 3-way-called me and said they had decided we needed agent names. I was Agent Fruit/Froot (I don't remember the original spelling), Leanne was Agent Bean, and I forget what Megan was. I decided to use agentfroot as my screenname for Neopets when I was 17-ish, and it stuck as my username for most sites.
Other non-boring-name-related screennames I frequently use:
blupyglet (AIM) - When I was 14-ish, I was trying to come up with an AIM name and was fiddling around with words and interests. For some reason, I decided to change the spellings of "blue" and "piglet," and I smushed them together. Of course, some people still think it's pronounced "bluppy-glet."
alexandriaSISP (hotmail) - When we were 13/14, a few of my friends and I had different names we'd use when talking online to people we'd never met. (The internet as we know it was fairly new to the public back then.) It was safer than revealing our real names, and we kind of created slightly different fantasy lives for these alter-egos. I was Alexandria (or sometimes Alx for short - the lack of "e" came from a typo, and the spelling stuck). The SISP stands for Smashing Icicle Sugar Power. The smashing icicle thing came from an incident when I was hanging out with Kathy one winter day, and while we were sledding, I started pulling icicles out from under a little bridge and then smashing them. Sugar power... well, let's just say I ate a lot more candy in junior high.
Clearly I stick to old screennames from junior/senior high. There have been several other ones I've used on various sites, but I rarely (if ever) use them. I used to use Superade on various sites back in 8th-9th grade. (I also had a Superade comic I'd draw during study hall.)