Needed a nice and unique username when i made a Travian-account a long time ago. I was a teenager and had an ability that few others around my age had, I liked coffee, and drank it everyday. I came up with Random Coffee, which I thought was a totally hardcore username, and even today, I think it is awesome. Not sure about the Random, but Coffee is because of my passion for coffee :P

My first ever forum was at BigFish and that was also my first gaming experience. I decided to sign on and a username was asked for, but never having done that before, I was taken by surprise. So I thought real fast and used my real name.
My Mother's name was Mary, her Mother's name was Anne. Mother was English and French, thus the French spelling - Marianne. My parents didn't want to leave out my Father's Mother, so it is my middle name. He was Irish.

Cassandra, might it have been, should you have gone for a female alias? Bit more tragic, though, as might be fitting with the world...

Actually that might have been an even more appropriate name, the only problem being that I'm not a woman (last time I looked anyway) ! Nobody ever takes any notice of true prophets in their own times anyway, so anyone who actually knows anything in these oh-so-clever (but not very wise) "scientific" days is bound to suffer in the same way as both Cassandra and Theoclymenus did : that's no great surprise ! Everything is, apparently, explicable in "scientific" terms - even the creation of the universe itself and it's ultimate demise. God, everyone is all of a sudden just so clever, so wise and so knowing about everything ! The internet isn't full of utter dickheads after all, then, it's full of geniuses !

I'm an old fart, and I've been using one version or another of Hetzer as a username since back when I got into Steel Panthers in the 90s.

http://en.wikipedia.org/wiki/Steel_Panthers

After a little research I found 6 prophets (or clairvoyants) mentionted in The Odyssey. Alithersis, Alkinos, Tilemos, Tiresias, Polifithis and Theoclymenos (Tiresias is the most noted). Cassandra is not in The Odyssey, she is in The Illiad, which is a different epic.
So, if you wanted a name from The Odyssey, you chose well. (I really like greek mythology!!)

As for the rest, I kind of agree with you. People can't tell the difference between information and knowledge.....

Well, the cat might now be out of the bag, but the Incredible Adventures of Sfoug Garistras/Kathar Istria surely sounds chuckleworthy.

And speaking of cats, this one inspired the onomatopoeia that is my nickname.

In my country if you name is Branislav your nickname will probably be Bane, unless you're Slovak by nationality then you're probably be Branjo.
"Hello" is my Counter-Strike (and other shooters) nick, I thought it would be hilarious that when someone checks who fragged them it reads "Hello", sort of mocking. :)
1+1=2

This is actually a really good topic and I hope to catch up on all the answers.

You must admit it's a pretty badass sounding nickname in English-speaking countries. :)

While writing the post I actually thought about how GOG peeps who are native English speakers will have totally wrong apprehension of how both of those nicks are pronounced. :D

http://www.youtube.com/watch?v=WTYmuHatJ14

Grim - ever since playing Grim Fandango (no joke) I used Grim as a nickname.

However, Grim is a widely used nickname, so when joining other websites, I was required to find something else.

When signing up on fruityloops, a website to compose music, I was trying to think of something "groovy". Grimgroove popped up and it's been my internet-name ever since.

I invented Forz. And it was already taken (must be my evil twin) so I chose lord becuase I IS TEH LORD!!!!!11! also it looks cool.

1322: Nothing special and a product of a lack of creativity. Essentially my initials in what they would look like using numbers instead of letters.

"13" in appearance, closest resembles a "B", 2 is the closest to looking like an "R." Technically it would be 131212 but I don't like all the ones, so I went with "2" instead of "12."

There ya go.