.io

A .io is chosen mainly by software companies and technical projects: a start-up whose product is an API, a team releasing a developer tool, a data company keeping its documentation and status page at the same address. Within that circle the extension reads as a technical product rather than a country code.

There are no registration conditions. Outside that audience the gain is small: a builder or a clinic gets nothing from it, and customers who do not know the extension have to be told how to spell it every time.