GameMaker Server is a GameMaker extension which allows you to add online INIs, online achievements, online highscores or online multiplayer to your game, without having to host your own server.
There are 907 developers using GameMaker Server, and 4,299 accounts have been registered. GameMaker Server hosts 1,162 games, and 106,872 logins have been made on 30,793 different usernames.
To use GameMaker Server, you'll need a GameMaker Server account, and you'll need to register as a developer. Every game has to be registered as well, before any of the online functions can be used.