I've been mildly annoyed that every component-based front-end library's Hello World is a counter, with a button to increment the count. But it seems like no personal knowledge management systems have a way to actually embed an arbitrary component like that, with its state. It would be actually useful for games and counting reps in various contexts.
We just opened signups for Cuttle, our browser-based 2D CAD project. Our beta community has made some cool designs for laser cutting, vinyl cutters (Cricut), pen plotters, and CNC.
The 4 points on the homepage are what set us apart from existing design software: live modifiers, reusable components, parametric design, and scripting. And being able to compose those elements non-destructively.