Disable Ads on YouTube By Enabling a YouTube Experiment
There are a lot of ways to block ads, but with a simple command in the developer console, you can disable all ads on YouTube via an experiment.
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 while it works, you get a lovely ad-free viewing experience without any plugins. It even works on those pesky video ads.