Sticky Notes is a browser extension that helps in taking notes for a site, that too on the site itself. The notes persists for future use and can be used to chalk down notes or important points for that page/site. These drag-able notes can be placed anywhere on the page, where ever it may be relevant. You can create notes via context menu or using shortcut keys. Usage guide is mentioned in the extension page.
I use it to chalk down action items from PRDs, Design specification, etc that I have to work upon. It is really helpful on github pages to note points for that repository like configuration option, current version which I am using for update purpose, pull-request/issue notes, or common usage cli command. I usually end up forgetting cli options for various tools, like youtube-dl, and pin frequent cli commands for them on the web.
Official Site: Sticky Notes