Made a tool to organize thoughts.
Actually it is a mind tree, but in a more web-friendly form.
It has pivoted from what I originally started building at evryca.com.
Some years ago I got the idea of fractal conversation, instead of old-school tree/ladder-like comments.
I wanted to see only comments related to the current level.
I started making "something" with fractal comments. This "something" was a project discussion platform. But it turned out that even I myself don't use it, and the idea of fractal comments stuck there unused.
And recently it dawned on me that it may be a conversation with yourself — thinking, brainstorming, taking notes, writing.
So made this kind of cork/whiteboard, where one can dive into the subject and, being in the flow, write and see only related notes and rearrange them later.
I'm trying to make it flawless and add keyboard shortcuts where it's possible (Ctr+Enter to submit idea, drag-and-drop to rearrange, Esc to jump level up).
So finally I've made a tool that I use myself and will update it gradually (sorting, touch devices, ex/import (json, text), boards, more navigation with keyboard and other stuff, and login).
A few suggestions about the UI:
Cards have no visual cue that they are stacked (I think this came up in some other replies here), so it is impossible to tell how much information can be found underneath. With books, for example, you can tell by their thickness roughly how many pages you can expect. But a general problem with digital content is that its dimensions can be infinite, especially if it is revealed/concealed interactively in a fractal-like manner as it is in your tool.
Personally, I would feel overwhelmed quickly after taking multiple nested notes, especially if I wanted to look up a piece of information that is hidden deep inside or if I wanted to reorganize my notes (although I think this is not possible yet in your app?).
I like that you made a breadcrumb-like navigation back to the parents - this helps a lot with orientation. But maybe also consider a more global navigation and a search tool to move more quickly between top-level notes and find notes that are deeply nested.
I would also adjust the font-size dependent on the amount of text in a card. If you enter a lot of text, cards quickly become too cumbersome to read.
I also think the parent card/header(?) should be differentiated much more against its child cards. I became a bit confused about which cards are clickable and which are not.
Nonetheless, I always appreciate people who build tools that they find useful for themself and share them with others, especially if they help with thinking.