Diogenes, a nick I’ve used elsewhere. And when that was taken, Diogenes the Cynic. Who knew? 
When I first signed up, I (rather lamely) used my e-mail nick, which is an abbreviation of my real name. Not so bright, rather uncreative, and you can’t pronounce it. So after a while I decided to change it in honor of the costume I wore for New Year’s Eve 2000. “Katie Scarlett” and “Scarlett” were both taken. (Bitches only posted a few times.) So here I am.
I wanted to use Dr. Bedlam, the nick I use on most other message boards and internet communications; it’s also my main email address. Unfortunately, the SDMB didn’t seem to like it. I tried Dr. Bedlam, Doctor Bedlam, Dr Bedlam, Doc Bedlam, Doctor B, Doc B, Doc Bee, and several other permutations…
…and it did not like them, Sam I Am.
So I tried several other nicks I’ve used over time. It didn’t like any of those, either.
I began randomly trying various ideas and nicks. “Wang-Ka” finally stuck. After quite a while. Not easy getting in here…
I wanted jim, but it was already taken. At least, now I think I just assumed it was already taken. If it isn’t, can I have it please mods?
I wanted just plain Pookah. Taken.
Then I tried Phouka. Taken, and s/he is still around.
So I went for this one which is a deliberate misspelling of a character from one of my favourite novels.
Actually this name is not what I would choose now. I chose it a good while ago and then didn’t post for ages. But I think I’m just starting to get to a point where people might actually remember who I am, so I’m sticking with it.
ReverseCowgirl says it all.
I wanted “JJ”, which I use on another board, but it was already taken. Never seen the guy post though, maybe he only hangs out in GD. On a couple other boards I ran up against a minimum name length, so “DoubleJ” has become the default I use when I sign up on a new board now.
This one. it was my e-mail address at the time, so I stuck with it. I think I may have wanted dead_paul, but that appeared to be taken. although who he is,???
I was considering changing to attackships , which is my newer e-mail, but everyone knows me as this now, so i’ll stick with it. I use that if I sign up to other boards (aside from NADS), as its nobody else uses it.
I wanted pandora, which is what I used to use pretty much everywhere.
Taken, so I went for belladonna because I was reading both The Hobbit and The Mangler at the time and had been struck by the inclusion of belladonna in both works.
Now I almost never use pandora (it’s almost always taken anyway), and answering to bella seems second nature. I’ve only ever seen pandora post here one time, and I think it was just a really old post I saw in a thread from the dead…
My first choice was jsgoddess. I’ve been using it since 1995 at various places, and it’s never in use. If I stumble across it in use, that just means that I’ve been there before and forgot!
Julie
lil_red_corvette. I didn’t even try it here though. Its been taken on everything else I tried, so I show no use.
Betcha can’t guess what kinda car I drive…
at least, I wish I drive 
I was Flora McFlimsey for about a year, till people started meeting me in real life and I figured it was silly to have a pseudonym.
Shortly before The Winter of Our Missed ContentTM, I signed up as Frank the Lost Ramone, my old college radio dj name. Then the crash came and wiped out every post I ever created with that name, aas well as my registration. When I tried to sign back up using that name, I was told it was too long. So I resorted to this name, the one I use when I’m gaming online or at laser tag.
I originally wanted to be OTTO. Unfortuantely that was taken, but on the positive side, at least the person who “stole” my name still posts.
Frank, but it was taken. (profile) He made 3 posts back in 2000 and then disappeared.
I use Frank everywhere I can as it keeps things simple, like Eve said.
Ever since I started BBSing in the late 80’s, I always went by Sir Blah. I registered here with that name years ago, but never got around to posting. When I registered my current name last year, I first tried getting on with the Sir Blah name, but apparently the system wouldn’t let me back on with it – not really sure what happened there. So, I decided to use my current, polar opposite handle. If a generous mod wanted to switch the name over I’d be grateful, but it’s not something I’m terrible concerned about.
I was so ignorant when registering that I thought “username” meant real name and that “signature” meant “the alias you will be posting under”. So , with that in mind , my first choice for username was what I briefly had as my (never-posted) signature - Clement T. Arglebargle IV.
Padre. It was my college nick name. But it was taken.
The moron posted to only 8 threads, 7 of them pimping his crappy band.
I’ve been thinking of petitioning the powers that be for the name.
Unfortunately for all, I got mine on the first try…
I got my first choice - I’ve never come up against someone else using it. If I were to choose another name, it’d be something that would be easy to shorten to a cute nickname. “Fairy” doesn’t really suit me. I’m more likely to answer to “Mom” - but I don’t intend to do any changing. I like my username.