Tribe Twelve/WMG

Tribe Twelve is what Alex Kralie's Marble Hornets would have looked like were it finished.
A man comes back to his hometown, witnesses how he's changed and how it's changed and how both have stayed the same. Poorly written, poorly acted, and Slender Man keeps appearing in the background.

Milo isn't dead.
Instead, he faked his death and is the one who delivered the package to Noah's house in Night Recordings.
 * Jossed by this picture:http://twitpic.com/7f7h35

The same person has been hacking all of Noah's videos somehow.

 * In Analysis, the final frame of the video has a hidden message: "Milo could see", referring to Milo's ability to see the Slenderman.
 * In Milo's Funeral, the last few seconds are footage Noah thought he deleted before uploading, of him presumably running from Slenderman.
 * In My Grandfather Karl, the end of the video has the hidden message "Not Safe"
 * In The Unboxing, hidden messages appear in the video one frame at a time:
 * "He knows what I have done"
 * An image of the Slenderman
 * "I will wait for you" with a picture of the Slenderman waving his tentacles menacingly
 * In Box Analysis, everything in the box has been delivered by this person, but the video in particular has "Do You Remember" hidden in it.
 * These are all from the same person or unknown third party. This party may have also scrawled the messages onto the tower in Submission 2.
 * At the end of 'Token Letter' there is the message "Observing You" next to the creepy eye symbol. Also of note, some of the letters in said letter have been darkened in--specifically M I L O. Which quite possibly supports the Milo isn't dead argument.
 * Confirmed when the Observer hacked Noah's formspring on 11/11

Noah's granpa Karl is being controlled by Slender Man
The facts: Add it all up, and it seems likely that Slendy made Karl tell that story to Noah.
 * Karl somehow knew about Noah's videos, even though no one told him.
 * Karl somehow knew that Noah was going to be nearby because of Milo's funeral.
 * Karl has previous exposure to Slender Man (Großmann) and knowledge of the Großmann myth (German version of Slender Man).
 * Karl somehow identified the man showing up in the videos as Großmann, even though he only saw Großmann once, at night, and man in the videos looks (so far) like a normal human.
 * According to the Observer, '[they] cannot touch him' because 'he has something of [theirs]'. So...
 * Actually, most of that could be attributed to him being part of the family (Families generally tend to gather for funerals and such.) and the fact that the Großmann terrified him makes it unlikely he'd forget it that easily. Plus, you know, maybe he was just poking around the internet and happened to find them one day. As for the 'can't touch him because he has something of thiers' part, however... well, this is a Slendy series. He probably is connected somehow if they say that.

Milo is the Observer.
Either he faked his death, or Slendy pulled some necromancy. The Observer seems very close to Milo, not to mention how many times Milo showed up in HELLOTHERE and the newest one, COMECLOSER. Plus, the observer is clearly shown in silhouette with nearly the exact same hairstyle as Milo. It's pretty obvious that it's him.
 * I don't think Milo needed glasses tho.

Slender Man is trying to make friends with Noah.
I'd heard the theory that he's just lonesome and trying to make friends (and accidentally cursing them/driving them crazy in the process, but it didn't seem plausible to me until I saw Tribe Twelve. Look at what he's done to Noah- he tries to visit him in his house, only for him to freak out every time he does so. So he gives him a cellphone and tries to call him. He also makes and sends him a lot of artistic video-letters, all of which are asking Noah to come visit him (maybe he's planning a party). He sends him another package of goodies, including a tissue for his nosebleeds (even with "Thought you could use this" written on it), and pills to help him calm down (Possibly Milo's psychiatric medication). In Nature Trail, he and Noah even play Hide and Seek! The portion where he starts filming even sums it up as such ("Hide and seek is fun [...] We enjoy playing.").
 * And if you take into the account that some speculate the Observer was once a human, then if could be that he and others in the collective are Slendy's 'successful' friendships.

The Observer is a sort of vulture-like scavenger that follows Slendy.
When we get our first look at him, he's clearly eating something. This is just after Milo drops his camera and disappears. If we assume that the bit where Milo O Ds on his medication is just a trick by Slendy to cover his tracks (which has been heavily implied throughout the series), it seems likely enough that the Observer is eating Milo. It's possible that while Slendy drives his victims insane and eventually kills them, the Observer follows along to feed on the bodies.

Noah is The Observer.
When the Observer took control of Noah's Formspring, he talked about how he used to be human, and how he doesn't perceive time "normally". Someone later brought up that, since the Observer does not perceive time linearly, the human version of him could currently be alive. But, more disturbingly, when the Observer was answering the questions, he comes across as a Deadpan Snarker. A lot like Noah. If the Observer ends up converting Noah to the Collective, this could establish a sort of paradox/stable time loop.

Slendy's one of those failed supersoldier experiments created by the Nazis.
First came to me while watching 'HAPPYBIRTHDAY' (For obvious reasons.) Okay, so Those Wacky Nazis were all about genetic perfection and shit, right? So who says they wouldn't have tried to make artificial genetically perfect soldier peoples? (It's not exactly uncommon speculation and may or may not have actually happened, but whatever. This troper's currently too hyped up on sugar to bother Googling anything.) So, as Artificial Human experiments that may or may not have Gone Horribly Wrong/Right have a habit of doing, Slendy eventually developed a mind of his own and/or the Nazis considered him a failure, and somehow he got unleashed upon the world, and so became the Eldritch Abomination most of us know and are scared shitless by today.

Noah's friend Kevin is the Observer.
Most of the evidence for this WMG comes from the latest entry, November 11th, which shows Noah's latest kidnapping from his point of view. Around five minutes in, the video distorts, and we are shown Kevin, who the description states is a friend of Noah's who helped translate Karl's german. Kevin has the Observer's frizzy hair, his square glasses, and holds up notecards written in the Observer's scribble style. Also, Noah's response to
 * Actually, Noah sees

Milo is Firebrand
With the other members of the Collective coming to prominence, most notably Mr. Scars and Firebrand, it could be very possible that they are also people that Milo knows, going off of the above WMG. Then add in the November 11 footage, where Noah very clearly recognizes Firebrand...