I've participated in a couple of js31kgames jams, including the very first one. I didn't do it this year, but it's certainly a fun little challenge. My favorite creation was a color matching game; I actually wrote a small post on the Intel Developer Zone blog about all the stuff I learned about color matching, color formats, and perception over the course of making this (it was just beginner knowledge, but much of it was things I hadn't even considered before).