GIFit! - Quickly turn YouTube videos into GIFs (chrome extension)
BriefTube - Instant video summarizer
Youtube Plus - Home of the extension YouTube Plus that allows you to experience more on YouTube - ParticleCore/Particle
SmartVideo - SmartVideo is a tool that offers users better experience on YouTube as it enables users to have better control over YouTube buffering, and playing options.
Turn Off the Lights - With a single click on the lamp button, the page will fade to dark.
Magic Actions - Magic Actions is a tool that comes as an extension for users to turn their computer as a home theatre for YouTube.