https://i.stack.imgur.com/vJCMR.png
It was made by a semi-famous founder I think that wanted to build the best mind mapping tool. It had a playful nature UI, not corporate. Very "basecamp" in tone.
You could link items between each other and embed everything from texts, urls, video, images, etc. You could see other people using the board in real time as well.
One thing that comes to mind that was distinct was that each card could have items, see the excalidraw image above.
Used it once at Papa, and never could find it again because of it's terrible naming!
Or an ER diagramming tool
Or UML-as-note-taking
I recognize none of those are the tool name, but maybe they'll help spur your memory :)