T̯̹͉̗͍ͅo̟͚̰̺͓̩̩ ̣̦̱͔͓̗̭͝i̜̦͝n̛v͉̖͔o̪͕͕̝̭͇k̬̺͖̖̖͢e͕̱̹ ͘t͍̹̩͕͉h̡̳̺e̸͉̮ ͏h̖̪̯̜͞i̤̱v̗̭̝̺ḛ̠-̬̮̭̟m̺̝̮̺͕͎͎ị̗̖̪̠͍̯n̹̳d̘ ͝r̗̮̰̪̺̪͘e̙̦͉̜͡p͎̝̬̻̕r̥̦̙̹̫̜͍e̯̳̗̙șé̗̙͓̣͍n̢͙̮t̟͎̜̝͍͙͓i̢n̘̭͠g̺̠͎̦̗͡ͅ ҉̮c̼h̸̥̯̝̼̭͓ą̬̬̗̦̠̠ͅo͉͟s̻̟. ̳̤͚̰͘I̱̪͙̟̖n͓̜̺̲͙v̙͖̼̦ơ͇̱̣̹̩̜ͅk̘͓͡ḭ̧͎̤̰͇̫n҉̗̥g̪̪̯ ̗̪͇̩̺t͏h̗͉͇͚͓e̺̫͓̫͔ ̗͍͠f̹͉͕̰e͉̦ȩ̘͚̰̜̭̥ͅl̤i̲̘̮͎͔n̻͈̫̱̲͇ͅg͓̩̙ ̴̫͍̤͖o̦̦̭̞̹f̷̞̳̩͔͔ ͉̯̖͚̣̻c̡͉̰h̛̬̞̱ao̪͡s̶̝̤̯̘̤͓.͇̦̫͟ ̸͉W̗͕͖̼i̷̟̭̹ṭ̹͚͉̜h҉̫ ̮̥̫̩̫̰̪̕o̵̳̱̤̝̤̹͓ư͎̥̜t̸͙̳̳ ͔o͕͜r̢̝̙̜ḍ͇͕e҉̣͖̤ŗ̰̲.̣ ͉͙ͅT͚͖̫̣͇͜h̢̻e̯̟̘͈̠̟͈ ͍N̷̝̜͕̘̯e̖͉̻͈̪̪z̨̺̜p҉̹e̜̱̜r͉̳̯͠d͏͈̘̟͈̣̤i̯͎a̹̲̗̯̝͔͈n̷͓̤̜͎ ̤̪̘̻h̟̱̠i̫̫̻̳v͇̘̠͓͇͖̺e͇̣͙-͈̠͇̭m̰̣͎͙̜̭i͚͓̩̫̞̟n͎̫̮̫̫͟ͅd͕͡ ͓͍͞ͅơ̠f̲̺̣͓̭̭̪ ͎͢c̛͉̙h̪͚̻̻̞̻͇͘a̗o͎̞̜̤̗s҉̱.͏ ͕Z̛͚̞̪̝a͡l͖͈g͡o͔̜̪.̮͚̖̬̬̩͖ ̤Ḫ̱̹͜ͅe͇̥ ̰͈̼͇̩̪͇̕w͉̻̠̳̠̩͟h̥̬͓̹̼͎̻o͓͎̙̣ ͇W̲̥̯̩̲a͔̰͈͔̫͎͕i͙t̫̺̼͉̥s̤͉͔̲̜͎͟ͅ ͍͓́B̺̩̙̲eh̖̥̜̙ͅi̜̙ṇ̩̤̮͉͘d͈̤̭͚̫ ̫͚͚̝͕͉͢T͕̰̥̯he̴͉͕̻͇̗̫ W͍ạ͍̺̠̞͜ļ̞̞̮̲l̤̤͠.̪̫̺̘̣̖͍ ̞͉Z̨̠̩͉͍̖̗A̶͖͇̼͇̥̙̝L̴̼̪̹̲̜̼̜G̶͕̺̩̭̳̬O͉̼̠͖̗̕!̰̺͇

http://eeemo.net is one.

That only does Zalgo, not the several other decorations shown in the app's screenshots.

I made http://lunicode.com, which does Zalgo, Flipping and other things.