Who is "A"?

I think A is Aria. 

Aria had a really strong motive to go after Alison given how much Alison had bullied her, not just with respect to her style but also with respect to her dad's secret. The texts began after Alison bullied Aria ruthlessly on Halloween. The texts stopped on the night of Alison's "death" while Aria slept. The texts began again when Aria was back in town a year after Alison's "death." And A hurt a lot of people, but for Aria, A only helped her romance with Ezra along and made it more public (which is what Aria wanted). 

Also, when Mona gains some control after going to the mental asylum, Aria has her first panic attack in the bathroom. So A's texts kept her stable, but Mona coming back strong really got to her. When Aria takes a flying lesson with the guy who used to give such lessons to the hiding Alison, you can almost see regret on Aria's face when this guy informs her of how troubled Alison had become from the various texts.

Aria has been playing with a lot of people quite openly. Whether it's lying to her parents about Ezra or manipulating them so she can hang out with him, flirting with Jason to get the scoop, or having some sort of control over her younger brother (though this control declines over time as her brother starts dating Mona), Aria has been almost as manipulative as A. In fact, the liars even said in an episode that Aria would be the most believable amongst them to be A, given how much she lies. I think Aria may have been blackmailing her brother in the beginning of the series to spy on other people - that's why her brother broke into so many houses - but once her brother got caught, she realized she needed to start using others, and do so anonymously. And that was the beginning of A's army.

PLL is also made by the makers of Gossip Girl [spoilers ahead!], so we can expect some parallels between A's and Gossip Girl's stature in the show. Dan, who was Gossip Girl, was a central character in that show, one who was part of the main cast and present in every episode from beginning to end. I think we can expect the same in Pretty Little Liars, and certainly any of the 4 liars + Alison would fit the bill. Secondly, Dan was shown on his laptop in the first few scenes of the show's pilot episode right after a character had wondered who gossip girl could be. Similarly, Aria is the one who shushes the viewers in the opening credit sequence for each episode. 

Lastly, Gossip Girl was about Dan's journey as the secret character - as one who struggles with making it to a prestigious college in the first couple seasons, one who is unable to integrate socially because of his financial background, and one who is viewed as an outsider by everyone. Gossip Girl was about how such a character was able to survive by starting an anonymous site and writing his way into elite society. Pretty Little Liars is not about the isolation that comes from being an outsider (which was Gossip Girl) - it is about the toll that bullying takes on the person being bullied. It is about the sense of shame that comes from having to hide your identity for fear that others will make fun of you. A is someone who has been forced to tell lies over and over again because she was afraid of the extreme bullying that Allie constantly put her through whenever A revealed her personality traits. One day, A woke up and realized that by hiding her identity completely (and not just lying about her unique personality), she could have an edge over Allie, and give Allie a taste of her own medicine. 

I think Aria fits this bill perfectly, because although Allie bullied all the 4 liars, her bullying of Aria was a little more ruthless - Allie loved Emily in a way, and thought Spencer's fighter spirit was fun competition, and treated Hanna like she were a doting fan and a fat, unconfident version of herself, but for Aria, Allie felt nothing. And the girls reacted to Allie's bullying differently - Emily was openly emotional about it and tried to make Allie understand her hurt feelings, Hanna felt insecure about her body image and concentrated her efforts inwards, and Spencer decided to fight openly. But what did Aria do? On the show, we see Aria doing nothing. Which is why it would make sense if Aria just made her battle anonymous as A.

A went too far, just as Dan went too far as Gossip Girl (Blair's and Chuck's accident). But Dan didn't kill anyone, and I'm fairly certain that A did not either. Just as Gossip Girl's negligence in sending out a message that others took too far caused Blair's and Chuck's accident, I think A may have sent out a particularly disturbing message to someone in the town, who took it too far and tried to kill Allie. And that may have caused A to feel too ashamed to come forward with her knowledge of the murderer. Or maybe A was afraid of the murderer's power - the coverup was quite substantial. Perhaps A thought that she could guide various people individually to solve the crime without revealing herself or her involvement in the process. After all, "never trust a pretty girl with an ugly secret."