I’m a huge fan of I use it in several ways, sometimes its prototyping CSS or JS without needing to build an environment, but mostly for exploring ideas and evaluating different techniques and libraries.
I’m sure at least two-thirds of my pens are forked, some with minor changes others unrecognisable.
I love the work of Karim Maaloul and his work with three.js and I just slightly adapted his Chill the Lion to create the Otis and Kiki Valentines animation.
My pens can be found here.