If you used some third-party sites to install Google Chrome Extensions, you will be unable to do it from next Google Chrome versions. Starting from Google Chrome 21, Google is planning to block installation of Chrome extensions from external websites.
If you try to install external Chrome Extensions, you will be redirected to Chrome Web Store and will be unable to install the extension even after downloading the .crx file.
This move is initiated because of repeated attempts by hackers to install and run malicious scripts in Chrome web browser to steal data and hamper users' privacy.
To be included in Chrome Web Store, developers have to adhere to Google's Terms and Agreements. Google will check each extension for malicious code before including them in the applications store.
It's clear that these steps will ensure better security from users' point of view. But what about those extensions that are not in the Chrome Web Store and are useful as well?