CONV. - Convert YouTube video to MP3, GIF or MP4
Enhancer for YouTube - Tons of features to improve your user experience on YouTube™!
Downie - Video download for OS X. Downie supports at this moment over 1,000 different sites.
Iridium by ParticleCore - Iridium is an extension built to improve the user experience on the new YouTube Material layout.
Yout - YouTube to Mp3 Converter
ImprovedTube - Allows you to improve and enhance Youtube visually or enable dozens of new features permanently or...