LoopMeUp от Wessel Meijer

LoopMeUp is a practice tool for guitarists. It runs on YouTube watch pages: mark the
spots that matter, loop between any two of them, change the tempo, and transpose
without changing the speed.

What it does:

• Marks. Save a named timestamp on a video. Type its time, or put the caret in the
  field and nudge it with the arrow keys — a tenth of a second a press, which is how a
  loop point actually gets set. Play from any mark without touching the loop.
• Loop. Choose any two saved marks as Start and End. It plays start to end, round and
  round, and keeps running while the panel is closed.
• Tempo, two ways. A percentage from 25% to 400% (the slider runs to 200%), or BPM:
  give the song's tempo, or tap it out, then ask for the tempo you want to play at.
  Pitch is preserved, so speed is never a pitch control.
• Transpose. Semitones, -12 to +12, tempo untouched.
• Tone. The same shifter by the hertz, against A=440. Type the reference the recording
  is actually at — 432, 415, whatever it turns out to be. No cap, and it shows the
  offset in cents.
• Keys, for when your hands are on the guitar. While the page has focus, [ marks the
  loop start, ] marks the end, \ starts and stops the loop.
• A panel, not a popup. It opens from the toolbar button and stays open until you close
  it; clicking the video does not dismiss it. It follows the video into fullscreen.
• Everything — marks, loop, speed, transposition and tone — is saved per video and
  comes back when you return.

Privacy: no data is collected, transmitted, sold or shared. There are no analytics and
no network requests to any server, and no remote code is loaded. Marks and settings are
kept in the browser's own local extension storage on your machine, keyed by video id,
plus one session flag for whether the panel is open. There is no account and no login.

LoopMeUp is not affiliated with, endorsed by, or connected to YouTube or Google.