Chrome on Windows already doesn’t allow extension installs from other websites except Chrome web store by showing error ‘Apps, extensions and user scripts cannot be added from this website’, you can still overcome it and install third-party extensions in browser by dragging and dropping crx extension in your computer onto chrome://extensions page.
Starting from January 2014 onwards this no longer works, Windows Chrome Stable and beta channels only allows extensions hosted in web store. However still there is a way: developer mode. Developer mode aimed for developers and power users allows install of unpacked extensions, here is how that can be done.
Installing Chrome extensions from outside web store in developer mode
1. Once you’ve downloaded desired extension from developer website, open Windows Explorer, Tools>Folder Options>remove tick mark for ‘Hide extensions for known file types’ and click OK.
2. Rename extension.crx to extension.zip and click ‘OK’ without worrying about warning showed on screen.
3. Extracts its contents to a new folder.
4. Now open Chrome, visit its extensions page, enable ‘developer mode’, click ‘Load unpacked extension.., browse for extension folder, select it and click ‘OK’.
That extension will be installed, all this may sound difficult for novice users but there is no way except this.
Chrome: ‘Disable developer mode extensions’
Chrome on Windows in beta and stable channels soon to warn for extensions installed in developer mode and asks users to disable them. Here is the complete text of that warning
"Disable developer mode extensions
Extensions running in developer mode can harm your computer. If you’re not a developer, you should disable these extensions running in developer mode to stay safe " with disable and Cancel options.
If you click ‘Disable’ all extensions you’ve installed using above procedure in developer mode will be disabled, you can still enable them later from extensions page. If you click ‘Cancel’ that pop up goes away without making any changes to chrome or extensions installed in it.
Thanks to Francois.