A punctuation mark most commonly used to show extreme excitement and/or shock. Since in most keyboards holding down SHIFT and pressing "1" creates an exclamation point, "!!1!" is used to imply that the writer didn't check what they were doing or that they were in a rush.
Person 1: Hey, little boy! You're mom's in the hospital. She asked me, a complete stranger, to come to your school while you were switching between classes and pick you up. So, wanna get in my car? I've got candy! Don't worry, I'm not a rapist!
Person 2: OMGWTFNO!!1!
Person 1: I JUST WON THE LOTTERY!!1! ZOMG!!1!!!1!
Person 2: NO!!1! FECKING!!1! WAY3Z!!1!
Person 1: Y3S WAYZ!!1!!!1! I JUST WON THE LOTT3RY!!1!
Person 2: ZOMG3!!1! HOW MUCH?!?!1?1!?!
Person 1: A MILLION DOLLARZ4S!!1!
Person 2: OMG!!1! YOU COULD BUY, LIKE, SO MUCH SHAMPOO WITH THAT!!1!
Person 1: IKR!!1!