Just because I'm rather bored at the moment, I'll not only explain my current name, but also some of the other names I've used.
When I got the internet for the first time, I had to get AIM since all of my friends were on it. At the time, I was into the card game Magic: the Gathering (I still am now, but not as much since I don't quite have anyone to play wtih). I focused heavily on black decks, destroying my opponent's creatures and taking their life points and whatnot. One of my favorite cards has always been
Necravolver because it's a fun black card and looks really cool. I decided to modify the name a little and drop the 'e' near the end to avoid running into anyone else online that might use the name. Unfortunately, I got annoyed over time as people called me "Necra", which sounded kind of lame to me, and people couldn't pronounce it to save their lives ("Nevacrawler" and "Neovolver" are two examples that come to mind). That little annoyance would eventually lead me to change the name.
For the longest time, I was addicted to TFC. Some friends had told me to go out and buy Half Life to play TFC with them. They even started a little clan of sorts; Nothing really official, but just a group of us to goof around and have fun. One name that had been in our group before me, but was never, ever active at all, was Grim. One day, my friend (who was technically the 'leader' of our group) cleared out all the inactive people, including Grim. I continued playing under Necravolvr for TFC, but when I got into Natural Selection, I decided I needed a new name. I tend to have a darker look on things, and I've always enjoyed morbid humor and irony. Grim seemed a little plain to me, and I have a little thing about how words seem to fit, a sort of ratio of length to height. Adding an extra 'm' to it added a bit of originality (although not much, as I've found out over time) and seemed to balance it out. It's easy to say, hard to mispronounce, and can't really get any shorter.
I still use Necravolvr for various things; I have a yahoo account under that name, my auth name on Gamesurge is Necravolvr, and some other items/accounts have it as well.
When the Half Life mod 'The Specialist' was released, I got into that right away because I had tried and enjoyed similar, but rather inactive mods like Opera. For some random reason, I liked the idea of creating another new name for this mod as well. Certain names that use unique letters that aren't used often, such as 'z' and 'k', letters with distinct sounds, appeal to me more for some reason. So the name Zodiac came to mind, which I used mostly on TS. I tried alternating to Zodiak, but it looked too odd, and the name Zodiac was uncommon enough to not become a problem running into other people that used it.
Along with the idea of unique sounding letters, I've always liked the name Ezekiel. It's sharp and smooth and just plain cool to me. Following a trend a friend of mine started to use "Savant" in names, his being "Savant Frost" after a sci-fi movie he mentioned about an evil supercomputer. I followed along, and my new AIM name became "Savant Ezekiel". Eventually, it became harder to explain the name to friends, and I wanted something that would fit my main online name better. Seeing as I spend a lot of time joking around and trying to get people to laugh, I created The Grimm Jester, which is my current AIM name, which is especially fun because it's kind of contradictory.
Sometimes you might catch me using a different name in game, either a variation of my current name (Grave, Reaper, etc), or one of my older names like Zodiac.
One name that I have under my control for AIM is Theo Diresong, which I've never bothered to mention to anyone because I forgot about it completely until recently. One day, bored in class, a friend and I began messing around with various anagrams, words that can be mixed up to create other words or phrases. Eventually, being the godless heathens we are, we decided to try the phrase "There is no god" which eventually transformed into Theo Diresong. I don't use the account, but it's there.
Cosmo has been slightly catching on recently due to Anarki discovering that my real last name is Kramer.
Finally, we come to the variation of my moniker that spawned from members of the LM community, although more specifically it was started by AgentOrange. Grimmus is simply adding the 'us' to the end of Grimm, but it's also similar to Grimace, which is basically a frown. Entertainingly enough, it's also the name of the big purple mascot we never see anymore from McDonalds, leading to many fun jokes and photoshops.
And that would be the history of every name I can think of that I've used.