The smallest Linux for Raspberry Pi?

There are dozens of articles similar to this, so I treat it just as another quick memo for myself:

1. You can create custom Linux-based system yourself with tools like:
   • Yocto Project
   • Buildroot
   • and probably many more
2. A ready out-of-the box solution is Core Linux / pi Core:
   • some info: here, here and here
   • very modular, add whatever you need
   • smallest version is claimed to have only 10 MB with GUI!
3. Smallest Rasbian OS is 0.5+ GB in size so is out of option.

And if anything above is not for you then maybe a… non-Linux solution for your RPi?