In universe I LIKE MEN AND SLENDER MAN!`
Slenderman in Marble Hornets
Slenderman in EverymanHYBRID
Slenderman in DarkHarvest00
Slenderman in MLAndersen0
Slenderman in TribeTwelve
UR MOM
Resources
Encyclopedia Slenderia contains large amounts of information on Slenderman canon.
The Slender Nation is a network of various places to discuss Slenderman, including a partly developed Wiki. They currently are most known for their interactions with people involved in Slenderman mythos, and in their developedment of Core Theory.
Unifiction is undoubtedly the largest discussion zone for the Slenderman mythos' video ARGs. Their Marble Hornets and Slender Man Mythos sections of their forums attract thousands of posts every week.