.sh

A .sh is used mainly where the two letters mean something in themselves: to developers sh is the shell, so an install script, a CLI tool or the documentation of a command line product sits well there. It also serves as a short tail for names ending in -sh.

There are no registration conditions. Outside those two cases there is little reason to pick it: a wider audience does not know the extension and has to be told how to spell it every time. Without a technical audience, something more familiar will serve better.