There are many extensions available for Google Chrome and Firefox to block Internet ads but the best I found is Adblock Plus, which is a great extension to block Internet ads, even YouTube video ads.
Adblock Plus works quite well when it comes to blocking those 10-30 seconds commercial ads that plays before the video. After you’ve installed this Chrome extension (Firefox add-on) you don’t have to watch those annoying video ads.
And not only YouTube, it blocks ads on other networks such as Hulu, CBS, ABC, NBC, Fox etc.
The Adblock Plus also has optional settings allowing users to disable ads, video annotations and auto-play too.