Those Rise of the Mushroom Kingdom Sprite videos are pretty awesome

Perhaps the "Mashed Taters" song and of course YTMND
Ebaums World's reputation for stealing stuff and getting paid for it
In the art community there's definately "internet art theft"
And arguably the biggest legendary-ish internet thing of all: 1337 5P3@K "Leet Speak"
Or at least the most annoying internet thing...