CodePen was supposed to be a weekend project.
Chris always wanted something like Jsfiddle/JsBin but with a simple way to see what everyone else was doing. He was making lots of examples on his site CSS-Tricks and he wanted something better.
He asked Tim and I to help him work on it. We did and we never stopped working on it because it kept getting more interesting.
Five years in and our weekend project isn't done. Quite a bit of scope creep if you ask me.