Mine is one I've been using on forums for years, and it's a character from the book series His Dark Materials. I chose it originally as my email address, and use it on forums because it's unusual and unlikely that no one else will have it.
My nickname in High School was Bubbles, so years ago I went into a chatroom where there was already a regular Bubbles, so as a courtesy, I quickly went and changed it to the first variation of the name I could think of. I'm glad to because I think Fizz really suits my personality.
Mine is simple. My friends have always called me a dreamer and I wanted it to be a little different so I just took out the second e. And the 802 is my birthday :-)
My username is my nickname but with an extra 'e' at the end. There's usually someone with the same nick, so I just add the 'e' at the end to make it unique. I've also been using this nick on forums.