Guess what I saw while waiting for youtube to load?
A request to inboxsdk.com.
What is inboxsdk.com?
e.g. you can build a chrome extension that shows a modified version of Gmail to the end-user. Whether you realize it or not, the is a whole world out there just building Gmail Addons, there are multi-million-dollar startups that are just Gmail addons.
Definitely worth checking out https://www.inboxsdk.com/