My new favorite thing is libraries that track every mouse move you make, every scroll action, every key you type, etc., including things you pasted into a textbox by mistake, and send it back over a websocket or something to a third-party service so that the website's UX people later can see how real people interact with their website and optimize it. Google for "website mouse tracking" or "website session replay" to see a bunch of startups that do this (many of whom have ads at the top).

I am incredibly excited for the GDPR to make these products too much of a regulatory burden to be worth considering.