I loved AI Dungeon for generating funny/stupid fanfiction style adventure/action/fantasy/scifi stories. It could keep track of characters and settings, have them return with motivations. And then something unexpected would happen. Provided me with a lot of entertainment.

But my problem with using it as a DM is that you have to police yourself. I could just declare that I did something and it would automatically succeed. There were no rules or antagonism in the system. It was more of a collaborative storytelling tool than a game.

To be fair I haven't used it in years so I dunno how it works nowadays.