I'm using "internet horror" as an umbrella term for any horror fiction that presents itself as, or is about, something "found on the internet." This can be anything from a story formatted as a blog post about something terrifying that happened to the author, to cursed image stories like Smile Dog.
I'll start:
Ted the Caver: Originally posted as an Angelfire site that has since unfortunately been taken down. Chronicles the journey of a caver who gradually discovers that the cave he's exploring has something living in it.
The Story of TRAINING.bsp: A blog reminiscing about the author's days exploring strange custom Half-Life maps with his clan. One of these maps takes a sinister turn.
The Dionaea House: A series of emails and text messages about a strange house. Or rather, a strange thing pretending to be a house.
I liked early Marble Hornets, though I remember a point where the people making it seemed to want to do more acting stuff and it declined pretty hard because the acting and character writing weren't great.
The middle chunk of the third act drags haaaaaaard. There's like 15 entries that are just all the characters running around the abandoned campus trying to find each other and it did not need to be that long.
I loved the very ending though, wrapped things up real nicely.
The first season of Marble Hornets is my favorite for how bleak and minimalist it is compared to the rest.