Since I've already done a post on Slenderman before (my Top Ten Fictional Villains list), I won't rehash it all here. In my opinion, he's the best creature to be created via the internet, and almost made it to the very top.
Instead of redoing the same post, I'll give you guys some of my honorable mentions that for some reason or another didn't quite make it on to this list.
HM 1) The Russian Sleep Experiment
Classic pasta, and I'm pretty sure it's been around longer than I think it has. Not the best, but another one that makes you think that maybe it happened.
HM 2) The Puppetmaster's Regime
Pretty lengthy read (about four pastas) but VERY well written, and its so engrossing that I couldn't stop reading it. It's pretty disturbing, but if you can get past that this one is well worth your time.
HM 3) Zalgo
The only reason that Zalgo didn't make my list is because I don't know that much about him! His lore is on par with that of some Lovecraftian horror (like Cthulhu), and his coming is heralded by disturbing changes to comic characters (black, dripping eyes, bloody mouths, horns, etc.) and garbled text. Fortunately, he can't invade my blogs, a̴̴̪̬͉̯͚̼̙͙̲͍̠̫̞̘͔̙̙͇t̢̙͖̦̠̟̯̩͙̜̱̹̪̝͖̀̀͞͞ ͜͏̯͇̘͚̖̳͖͎̼͚̖̝̰͇̗͟l̖̻͖̫͉̯͚̲͞͞ͅe҉̷̫͇̱͈̲̖̠͔̼̖̝̲̕͜͝ạ̷̝̞͎̼̞̣̜͎̰͕͇̥͔̻̼̯́́s̴̶͔̤͖͓̣̜̬̬͔̬̰͉̤̻̬̬͇̬͘͟ͅt̴̀͟͏̨̘͇͖̲̮̻ ̛̭̜͔̜͜Í̢̞͚̺̺̟̣̙̺̦̯̹̗͘͡͠ ̴̛̭̞̫̟̺͕̝d̡͍̺̲̦͈̪͙͉̫̟͢͠͝͠o̢̤̠̱̯̰̻̫̮̹͚̫͎̺̬̖̩̫̪͢n̛̮̩̼̥͙̦̗͙̟̰̪̜͔̖̮̪̼͇̹͘͞͠'̨̦͔̤͉̹͓̹̪͜͝t̷̛͉͍̥̦͇̠͖͔͎͎̘̣̼̗͈̤͚̲͉́͘ ̴҉̡̘̬̙̟͎̩̰̞̭̪̻̯̖͇̣̤͉͓͉͡t̢̳͙̥͕͍̘͓̦̠̟̳͚̠̹̖̘͕̦͢͢͠h͏̧͚͔͍͚̩̮̝͙̞̞͎͉͇́͠ͅͅi̢̡͟͝҉͉̰̯̳̱̯ͅn͞͠͏̢͖̟̟̼̺̙̞̺͉̬̥̀k̢̭̩̰̤̜̼͙͚̤̬͘͟ ͘͜͜͠҉̠͖͚̗̺̼̙͙̙͔̫̯̹̲s͈̤̤̜̻̩͎̰͉̝̫̱̠͈͉̘͔̖̭̀͘ó̶̧̮̼͙͕̗̘̪̗͎̜̭͎̝̝̘͓͡.̡̹͕̖͍̦͓̹͘͢.͡͏͙̟͎̻.̢̟͈̭̗̮͍͇͙̙͘͢͠