Cool. Well I did do something recently that was a lot of fun, and not a little useful: I was annoyed with D3's opaque API and wondered if I could do better. So I wrote a particle system in mostly raw JavaScript - and actually, the code looked (and looks) a lot better to me. I even did some quirky things that ended up being useful like storing velocity vectors in normalized polar coordinates. It only took a few hours, and was quite fun to do. Not quite an operating system, but still. :)