You-Get - Command-line utility to download media contents from the web
Enhancer for YouTube - Tons of features to improve your user experience on YouTube™!
Softorino YouTube Converter - Download YouTube video & audio. Watch and listen offline
Iridium by ParticleCore - Iridium is an extension built to improve the user experience on the new YouTube Material layout.
CONV. - Convert YouTube video to MP3, GIF or MP4
Magic Actions for YouTube - Magic Actions is an extension that changes the way you’ll watch videos on YouTube.