Awesome

Chrome extension source viewer (CRX Viewer)

View the source code of any Chrome extension in the Chrome Web store without installing it.
https://chromewebstore.google.com/detail/chrome-extension-source-v/jifpbeccnghkjeaalbbjmodiffmgedin

Also available for Opera and Opera's add-on gallery.
https://addons.opera.com/extensions/details/extension-source-viewer/

And also available for Firefox and addons.mozilla.org (WebExtensions and other Firefox addon types). https://addons.mozilla.org/firefox/addon/crxviewer/

Online demo (select a .crx / .nex / .zip / .xpi from your disk or a URL to try out the viewer):
https://robwu.nl/crxviewer/

Features

This Chrome extension adds a button to right of the omnibox when a CRX file has been detected. This button appears in the following extension stored:

Chrome Web Store
Opera add-ons gallery
Firefox add-ons gallery
Thunderbird add-ons gallery
Microsoft Edge Addons store

Upon clicking the button, two actions become available:

Download extension as zip file
View source

The default action (showing the above options) can be changed via the "Primary action on click" menu that appears when you right-click on the extension button.

The "View source" option opens a new tab with a simple viewer, with the following features:

Download-as-zip and download-as-crx at the upper-right corner.
List of file names, and the option to filter files with a regular expression.
Find files containing a string, or with content matching a regular expression.
Quickly jump between search results, or from/to a specific line.
Automatic beautification (formatting) of code
Syntax highlighting
Image preview
Show hashes (md5, sha1, sha256, sha384, sha512) of the file content.
View content of embedded zip files.
Download Chrome Web Store extensions for a different platform (e.g. Chrome OS or NaCl).
View the contents of any URL or zip file.
Permalink to file and search result within a zip or extension file.

As mentioned before, this extension activates on the Chrome Web Store by default. It's also possible to view the source code of Chrome, Opera 15+ and Firefox extensions that are hosted elsewhere. Further, there is an option to show a contextmenu entry on links whose target is a Chromium extension. Both features can be toggled at the options page.

Copyright

This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.