I can't find the website, but there's a site that specifically tells you everything it knows about you just by keeping the page open.

It tells you if it knows where your cursor is, what pages you've been to, what your computer is, etc.

It was intended to show that you, the user, give out a lot of data without even realizing it.

EDIT: I did a search on reddit and it looks like it actually is https://clickclickclick.click, but because the site hasn't been accessible I can't be sure.