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 854 developers using GameMaker Server, and 3,728 accounts have been registered. GameMaker Server hosts 1,105 games, and 96,384 logins have been made on 28,290 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.