There are a lot of different ways to block ad online. One way to disable all ads on YouTube insolves an experiment with a simple command in the developer console.
Google frequently tries out new features with experiments via TestTube. A less advertised experiment can disable all ads on the site. To enable it, you'll need to enter the following code into the developer console (Ctrl-Shift-J for Chrome, Ctrl-Shift-K for Firefox) in your browser while on YouTube:
document.cookie="VISITOR_INFO1_LIVE=oKckVSqvaGw; path=/; domain=.youtube.com";window.location.reload();
Just paste that code, press enter and the page should reload. Boom. No more ads. Since this is something that Google is allowing, it's possible it could go away in the future, but you get a lovely ad-free viewing experience without any plugins while it works. It even works on those pesky video ads.