I know, it’s probably been talked about before. So sue me.
I’ve always liked (off the top of my head):
You With The Face
Shagnasty
Stink Fish Pot
mmm
Exapno Mapcase
Qadgop the Mercotan. I’m not fool enough to think even a pure neutronium hull can stop him.
Also,
Bell Rung, Book Shut - Candle Snuffed
mmm
Left Hand of Dorkness
I kicked myself when I saw it, because I loved that book, and absolutely hated this one.
Least Original User Name Ever and the fact it can be acronimized as **LOUNE
**
Really Not All That Bright.
Covered_in_Bees!
and
Shot From Guns
The names, not the actions.
Chicken Of Bristol
Always get a chuckle when I run across No Umlaut for U
Patty O’Furniture
Enola Straight
I always liked Friscalating Dusklight. Haven’t seen him/her around for quite some time.
Typo Knig. I laugh every time I see it.
**An Gadai **- I have no idea what it means, but I picture many men on horseback cresting a grassy hill yelling AN GADAI!!! as they attack the village below. Every time I see that name, that is what I think.
It means “The Thief.”
Cheez-Whia. Not jealous, per se, but it makes me giggle, especially since I remember when that poster joined and said it was the result of a typo. I say it in my head as “WHEEE-AH!” all enthusiastic-like.
I always liked
Bob Loblaw
But, you have to say his name out loud to get it.
I like pulykamell and punoqllads. I always think the former should be able to be read like the latter, but llawekhlud doesn’t make much sense. Turns out it’s Hungarian for turkey breast, but it sounds more like advice for Egyptian artificial inseminators. 
P.S. Bob Loblaw doesn’t work for those of us with British accents. I had to have it explained to me…
Not really jealous of any as in, “I wish I’d thought of that!” I do wish I’d spent more time thinking about a name before picking one, though. I’d have at least spelled Rachellelogram properly (see: parallelogram).
I’m sure the powers that be would change it for you.
I’m jealous of Freudian Slit’s username.
Among posters still around, Czarcasm is the one I wish I could steal.