Adblock Plus (Firefox)
Blocks annoying video ads on YouTube, Facebook ads, banners and much more. Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
Adblock Plus allows you to regain control of the internet and view the web the way you want to. The add-on is supported by over forty filter subscriptions in dozens of languages which automatically configure it for purposes ranging from removing online advertising to blocking all known malware domains. Adblock Plus also allows you to customize your filters with the assistance of a variety of useful features, including a context option for images, a block tab for Flash and Java objects, and a list of blockable items to remove scripts and stylesheets.