According to a recent post in the Chromium blog, Google will be removing the ability to install 3rd part add-ons in future versions of its popular Chrome browser. Starting in January, 2014 Google will require all extensions to be hosted in the Chrome Web Store with the only exceptions being for enterprise and development testing. This change will affect the stable and beta channel of the browser. If you want to ignore the change it will require switching to the developer channel.
Google’s reasoning behind the change refers publicly to security concerns with malicious extensions. In particular, one of the main reasons is that it will block the installation of bundled crapware that comes with installers from places like CNET or Sourceforge.
More information on the Chromium Blog.