Marginstone
I’ve been building a small Mac app for keeping passages from things I read. It is called Marginstone. The idea came from Varun’s Buddy, which I acknowledge with delight and eternal gratitude.
A small companion sits on the screen, and you drop passages onto it. These are saved locally; you can edit them, organise them by source, and export them later. There is also an option to publish them to a website, though you need neither a website nor a GitHub account to use the app. The companions are fossils: a nautilus, a leaf, an ammonite, a brachiopod, and a fern. Fragments preserved, I suppose, although I also simply liked how they looked.

There are rough edges, particularly around identifying sources and capturing from different browsers. If you try it, I would be interested to hear what works, what doesn’t, and what seems unnecessarily complicated.