YouTube Distractions Remover
Userscript (Recommended)
Bookmarklet
For those who can't use the userscript, simply save the bookmarklet and click it when you watch a video to prevent yourself being distracted. This bookmarklet hides the sidebar, guide, comments and search box from any YouTube video.
To save the bookmarklet you can either drag it to the bookmarks bar in your browser or right click on the link and select 'Add to bookmarks/favourites'