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A site where you can send push notifications to other random users
(randomping.pythonanywhere.com)
You can enable push notifications for the site, and then you're able to anonymously ping other random users, even if they have closed their browser (it's pretty addicting). Opting out is easy, and no sign-up is required.
Noone has pinged me yet :(
Unfortunately, not that many people have signed up yet. You can stay signed up though and get surprised when you receive a random notification once a month or so (it's fun!) ๐