Mine is a somewhat long-ish story:
Back in the day I used to share nicknames with Moach, my brother... Since we shared the 'puter, it was easier to just use a single nickname so we wouldn't need to be constantly logging in and out... So I used to be Moach (which is a word we made up... but that's a story for him to tell)
Anyways, We used to have this heavy metal band, which back then was called "Bad Harvest"... So whenever I needed an alias for some online thing, I would use "TheHarvesteR"...
After I moved out and got my own computer, I took on "TheHarvesteR", and over time it became just "HarvesteR" (I'm still TheHarvesteR when HarvesteR is unavailable)...
Whenever both are unavailable, I tend to go with my alter-ego, PsykomantaBrain.
Cheers