uxn
uxn is a virtual architecture invented by Devine Lu Linvega.
It was created as a small platform for experimenting lightweight and sustainable software development (especially because its main creator lives on a boat, without access to a lot of electric power nor network bandwidth), but also to serve as a simple common ground for archiving software (because code based on modern complex architectures is extremely prone to decay and obsolescence: once the machine is gone, the software is gone too).
The main manual on how to program on uxn can be found here.