YouTube Quickmark от Florian Klar | Klar Web Services

YouTube Quickmark is a tiny, no-frills bookmarking tool. While watching any YouTube video, press B to place a blue marker at the current spot on the timeline. Your markers are saved per video and come back automatically the next time you open it. Click the toolbar icon to see all markers for the current video as timestamps — click one to jump straight there, or remove markers individually or all at once. No accounts, no notes, no data collection: everything stays in your browser.