I think I've picked my next side project. It sits nicely on the border between pointless and useful, and it's something particularly interesting and profitable for me. I've been thinking about how to do it, and I'm pretty sure I have a good approach. Best of all, it isn't anything I've mentioned here yet. The only thing is, it will take some focused effort.
One more side project idea:
Blog about the cool ideas that own my brain. I suspect there are a few really killer counterintuitive ideas I've run into that really knocked me flat and guide my entire philosophy—reliable systems made of unreliable parts (i.e. the Internet); order without design (the most important idea in economics and biology); etc. The main question is whether enough of them are non-math and non-computing to make them worth writing about.