MegaZeux: Game creation system released in 1994 and still being developed today

It has a number of problems, but there are a few games with fairly active communities that are built on top of BYOND, like Space Station 13. [1]

[0]: http://www.byond.com/ [1]: https://spacestation13.com/

    Overview of MegaZeux

    ...

    As you may already know, MegaZeux is a game system which
    allows you to play almost limitless worlds with
    dated-yet-charming graphics and with excellent digitized music
    and sound. Not only are there several MZX worlds out there
    already, but new worlds are being uploaded to large websites
    like DigitalMZX. However, the best feature of MegaZeux is the
    World Editor.
    
    Using the World Editor, ANYONE can create the world of their
    dreams. Make it as simple or complex (well, almost), as easy
    or difficult, as long or short as you please. We aren't just
    talking about worlds made up of petty, pre-programmed enemies
    and objects; MegaZeux has its own, easy-to-use PROGRAMMING
    LANGUAGE called Robotic that allows you to create objects,
    engines and worlds that do almost anything you desire. Not only
    can you make your own game with the editor, but every world
    MegaZeux can play can be opened in the editor. Take a look at
    how other games tick!
    
    For the newest user, it's recommended that you play Caverns,
    the first ever MegaZeux game, to get the feel of simple yet
    well-designed games in MegaZeux. You may wish to read the help
    section entitled "Controls" to learn how to play MegaZeux.
    
    If you're more adventurous, start with a more complex game like
    Demon Earth or Bernard the Bard, or a prettier game like & or
    Fritz Blitz to see what kind of graphics functions, bells and
    whistles MZX can offer.
    
    Once you have the feel for the game, feel free to dive into the
    World Editor and get messy! You should probably read the help
    section entitled "The World Editor" first.
    
    To go to one of these sections now, hit Enter after aligning
    the arrows with one of these choices. Press ESC now to exit to
    the game.

There’s something about the programming model (“robots” so easily model people conversing and doing silly things) that allows you to just casually sketch out whatever’s in your head. That’s given rise to so many quirky, funny games for this system in a style that might never be made in any other context, except of course its predecessor ZZT. Dive into the archives and have fun.

The community, to my knowledge, still meets up yearly and also has impromptu hour-long game jams.