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. Here's how to turn it on:
- Open up a YouTube video (any will do).
- Open up the developer console (Ctrl-Shift-J for Chrome, Ctrl-Shift-K for Firefox)
- Enter the following code:
document.cookie="VISITOR_INFO1_LIVE=oKckVSqvaGw; path=/; domain=.youtube.com";window.location.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.