I freaking love Nyan-Cat.
So dumb, yet so cute and addictive. It's a cat...with a pop tart body...flying through space via a rainbow. All in 8-bit, nyaning all the while. It's kinda awesome. To me, at least. To others, it's a test of endurance before your thoughts turn into nyans and then you can't stop nyaning so you have to nyan your nyans. And as of this writing, the original video has over 31 million nyans.
And like all viral videos, there were numerous knock-offs, many of them just as funny as the original. That's kinda rare, but "Nyan" is pretty much universal.
Here are some of my favorites:
Super Mario Bros.
And, of course, a 10 hour long version.
If I had any skills as an animator, I'd find some way to sync this up with the Clarissa Explains It All theme song. I mean...come on, Youtube!