Try to block annoying embedded ads in YouTube videos.

block-youtube-ads.js

chrome.webRequest.onBeforeSendHeaders.addListener(

  function(details) {
    if (details.url.indexOf('googlesyndication.com/pagead/show_ads.js') !== -1 ||
        details.url.indexOf('googlesyndication.com/pagead/show_companion_ad.js') !== -1 ||
        details.url.indexOf('s.ytimg.com/yts/swfbin/ad') !== -1) {
      return {cancel: true};
    }
  },

  {
    urls: [
      "*://*.googlesyndication.com/*",
      "*://*.ytimg.com/*"
    ]
  },

  ["blocking", "requestHeaders"]

);

manifest.json

{
  "version": "0.0.1",
  "manifest_version": 2,
  "name": "Block Youtube Ads",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "*://*.googlesyndication.com/*",
    "*://*.ytimg.com/*"
  ],
  "background": {
    "scripts": ["block-youtube-ads.js"]
  }
}