Roblox

Roblox is best understood as a distribution platform with a game engine attached. The catalogue is entirely user-created, which means quality varies from genuinely accomplished to unplayable, and the discovery problem is real, the front page is driven by concurrency, so finding the good ones takes some deliberate searching.

What makes it worth listing is that some of those user-created games are properly designed and have player counts that would make them successful standalone releases. The creation tools are also the most accessible route into actually building a multiplayer game that exists, which is a reasonable argument for it on its own. Be aware that the economy is built on a purchasable currency and that a lot of the catalogue is designed with children in mind.