StoryNexus supports authors in building what Failbetter calls “quality-based narratives” — stories where the available nodes and choices depend on the player’s stats at that point in the narrative. The result is somewhat more fluid than the typical choose-your-own-adventure model, in that it doesn’t impose a strict order to the branch points, but allows the player to explore whichever of the available storylets happen to be open to him.
The only other tool I know of that currently supports this particular blend of world model and choice-based storytelling is Varytale — not entirely coincidentally, since Failbetter had considerable input about Varytale at the design stage. But where Varytale tends towards the more literary possibilities of the format, with long prose passages and a book-like presentation, StoryNexus aims more at RPG-like explorable worlds reminiscent of the original Echo Bazaar. The tool allows authors to select from artwork and theme options, and in the future will also allow authors to upload their own imagery to accompany their creations.
StoryNexus authors may offer their work for free, or may charge for it, as they like, using StoryNexus’s built in system for putting purchase gates on content. (This functionality is I think closed at the moment, but scheduled to open very soon, offering a 60/40 split with authors on any revenue a story may earn.)
Edited to add: there’s also some coverage at the Escapist.