Chrome: From next year, Google will block the installation of browser extensions that don't come from the Chrome Web Store. While this change is only officially impacting Windows users, in reality than means few extensions will be able to attract an audience without being within the store.
The change has come about to avoid extensions installing silently and making changes to users' configuration without letting them know. Chromium engineering director Erik Kay explained the logic in a blog post:
Since these malicious extensions are not hosted on the Chrome Web Store, it's difficult to limit the damage they can cause to our users. As part of our continuing security efforts, we're announcing a stronger measure to protect Windows users: starting in January on the Windows stable and beta channels, we'll require all extensions to be hosted in the Chrome Web Store. We'll continue to support local extension installs during development as well as installs via Enterprise policy, and Chrome Apps will also continue to be supported normally.
While the restriction only applies to Windows, Chrome extensions are rarely platform-specific, which means any serious extension developer will have to offer the store as an option, even if they also include non-Web Store versions for Mac and Linux.
Protecting Windows users from malicious extensions [The Chromium Blog]