What are some of the coolest things you've stumbled across on the the internet that other people should definitely check out?

For me, two random things to lead off:

1) There's a web serial called Worm that is the equivalent of a series of books; it's one of the best depictions of superheroes and one of the best "worlds" I've ever read. It's long, but if you're looking to dive into something, it's absolutely worth a read!

2) This three minute video of zooming out from a person to the entire universe and zooming back inside her eye: 

https://www.youtube.com/watch?v=CWGWO2thgqw

What do you know of that's worth sharing?