The GettingStarted extension adds a Special:GettingStarted page which can present introductory content and tasks to newly-registered editors. This extension is constantly evolving as part of the Onboarding new Wikipedians project.
This extension is being used on one or more Wikimedia projects. This probably means that the extension is stable and works well enough to be used by such high-traffic websites. Look for this extension's name in Wikimedia's CommonSettings.php and InitialiseSettings.php configuration files to see where it's installed. A full list of the extensions installed on a particular wiki can be seen on the wiki's Special:Version page.