I've been watching the marathon like the loyal fan I am, and I came up with a few events which are leading me to believe A is blackmailing other people:
-Mike punching Ezra; he seemed to be acting very strange from when he came down the stairs. He never said anything and then just randomly took a swing. (A is probably who he's been talking to on the computer, as others have pointed out)
-Noel breaking up with Mona, and possibly the subsequent hookup with Jenna (to make him look suspicious to the Liars)
-Maya "torpedo"ing the dinner with Emily and Pam- perhaps the ex-boyfriend actually doesn't exist, and A texted her in the middle there to remind her to bring it up so as to cause friction (A as the "stalker" has been a popular theory)
-Kate sending out the text from Hanna's phone, as to me it seemed like Kate didn't really have a trick up her sleeve she was waiting to pull out, even after Spencer confronted her
-This one's a bit of a stretch, but Caleb continuing his pursuit of the phone's information after Hanna told him to stop. I think A knows that the Liars won't find anything extremely revealing on the phone, and therefore used this sugar bowl to drive a wedge between Hanna and Caleb. I feel like Caleb wouldn't continue without Hanna's consent; doesn't he trust her more than Emily/Aria/Spencer? A's threat to Hanna to make Caleb stop hacking was to reveal the money in the ravioli box, which I feel like A would never reveal, as it would change the status quo for Hanna too much; she'd probably have to move away if her mom went to jail
-I think that at the school lock-in, A told Noel that Caleb was up to something (hurting Aria? I think Noel still has feelings for Aria, which is another reason why his hookup with Jenna is suspicious), leading to his fervent search for him
ETA: Maya lighting up at Emily's party
Mona helping Emily? In an attempt to make Emily suspect her
I find it highly possible that nearly every side-plot is a diversion directly set up by A, through manipulation of these other characters.
Edited by MahoganyBonics, Feb 20, 2012 @ 7:42 PM.