Briki WP Registration Whitelist
This is a WordPress plugin I wrote back in April of 2011. It was a request from a user in the Greek WordPress Users' Forum (wordpress.gr).
This plugin matches a new user's email address against a whitelist of domains, during the user registration process, and only allows the registration if the email address is under a whitelisted domain. It was written for whatever version of WordPress was current back in April 2011. This blog post (in Greek) was the "release note".
This plugin is licensed under GPLv2.0, same as WordPress was back then.