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 886 developers using GameMaker Server, and 4,026 accounts have been registered. GameMaker Server hosts 1,142 games, and 102,727 logins have been made on 29,735 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.