ExtendTube

This extension extends YouTube player capabilities. It will add few buttons on
page that will allow you to: loop video playback (whole or selected part), seek
video back or forward, step one frame forward, download video, choose default
video quality... You can also control video via keyboard shortcuts or extension
pop-up.

FEATURES (almost all can be configured):
— hide ads on page
— hide in-video ads
— disable autoplay
— force buffer whole video
— loop video (or selected part)
— force video quality
— prevent cross play
— autoplay on page focus
— enable wide player
— autohide video controls
— hide video annotations
— 'pop-out' darkens page and centralize video
— download video (SD, HD, 3D, WebM, Mobile)
— download just audio (MP3) - see help
— load lyrics for video (from lyrics.wikia.com)
— convert thumbnails to videos (preview)
— show video ratings above thumbnails or in description
— keyboard shortcuts
— hide some parts of page
— change page colours
— change player theme
— add custom CSS
— add custom JavaScript (Actions)
— detect videos embedded in other sites (only via iframe)
— watch HTML 5 videos in full screen (F11) - see help

OPERA 15:
Working on it. May take a long time, this extension is big. This project is on GitHub, check the links at the right and you may help me convert this.