Acme: http://acme.cat-v.org/ It can do everything Emacs can but it's so lightweight it almost doesn't exist. Its power comes from the fact that it brings your external tools together.

It can be programmed using its filesystem (9p) interface, and extended by adding plumber(4) rules.

A demo by Russ Cox: https://youtu.be/dP1xVpMPn8M