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 955 developers using GameMaker Server, and 4,562 accounts have been registered. GameMaker Server hosts 1,198 games, and 114,274 logins have been made on 31,630 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.