I'm not going to go into a lot of detail, as I've posted this over and over and over again and it takes forever to type
walnut100 - Inside joke. Name spawned from 6th grade English class (Worst class EVAR BTW) The 100's just a nice number, unlike most numbered usernames.
walnut1002000 - Is basically walnut100 with a code. The 2000 comes from 100, advanced by 1, multiplied by 10 so you get an extra digit.
Cl0ud - Chat alt nick, came from a joke in the chat about l33t Final Fantasy 7 characters.
N-Gage - You bought one, didn't you? Inside joke spawned around the time I was opered on Windfyre. My newfound ability to mask my host caused N-Gage >_>
Mewtwo454545 (I think) - Don't ever call me this one, it isn't really a nick. This was my first ever online account name, specifically on Neopets. (*shudder*) I went with the generic crappy name based on something popular like a lot of internet users at the time. Just threw it in there because I did use it for a few months...