Gossip Girl/WMG

Gossip Girl is Veronica Mars.
Someone has given her a long-term, ongoing PI job to collect information on all the people in the UES, and figured this was the easiest way to get mass info on the children.

Gossip Girl is not one person, but rather all the main "teen" characters.
"The Goodbye Gossip Girl" aside, all the characters send in tips and take turns editing the Gossip Girl blog and sending out alerts, which explains why the characters are seen around laptops most of the time. The blog may also be the unifying link between the characters currently stuck at NYU (Dan, Vanessa), the kids from the UES and Jenny, since the show makes no effort to tie in their stories any more.

Gossip Girl is Samantha Barrish.
Using her state of the art computer, she spies on the Humphreys and the UES teenagers as "Vanessa Abrams" and works on Gossip Girl the blog from her secret apartment in Brooklyn. How and why she left Miami for this job is not known.

Gossip Girl is Chuck Bass.
If it is one of the recurring characters, then it's got to be Chuck. A few reasons:
 * Chuck is the only one who's never really been damaged by Gossip Girl's posts. She's gone after everyone but Chuck. The only blast that really hurt him was based on something we saw him write to the blog which backfired on him (back in A Thin Line Between Chuck And Nate).
 * When Gossip Girl got all characters to go to the same bar in the season two finale and then sent them all a text, Chuck was the only main character who wasn't in the room. He was, however, standing right outside. He was also sitting next to Serena with his phone in his hand when she started writing the text to expose Gossip Girl earlier in that episode, which meant that he had opportunity to turn the sound off on his phone).
 * Gossip Girl seems to be the only one who always shares Chuck's level of faith in the Chair pairing.
 * None of the characters are around to witness everything that ends up on the blog. Chuck, however, has private investigators on speed dial. If he's anything like his father, then he uses them to dig out the secrets of people close to him. Posting those secrets on a blog seems right up his alley.
 * In the episode There Might Be Blood, Chuck gets a bogus report posted on Gossip Girl which, according to Blair, is a difficult thing to do. He replies he would only do it for her.
 * In It's A Dad, Dad, Dad, Dad World, Blair gets a Gossip Girl blast via text, outing that she's not transferring from NYU to Columbia. Chuck makes a jab at her for being stuck at NYU rather than an ivy right before she gets the text. When she reads it, he guesses that it's a Gossip Girl blast without having much indication that it would be. The tip was sent in by one of the Columbia girls who overheard Blair talking to Nate; she, Nate, and Gossip Girl ought to be the only people who knew, but Chuck seems very much in the know.

Gossip Girl is Nate.
Think about it. People tell him nearly everything about anything, all because they think he's too dim to understand it. In reality, he's a super genius who secretly exposes his "friends" to the world, all for the satisfaction of knowing that he is not one to be underestimated. Ladies and gentlemen of the jury:
 * Whenever he's reacted to a major GG blast (Blair's "pregnancy" in A Thin Line Between Chuck and Nate, Serena and Dan's morning after in Last Tango, Then Paris), there's been a witness. He puts on a surprised face to sell it to them, something that he would have no need to do when alone.
 * In The Goodbye Gossip Girl, he's with Serena when she texts Gossip Girl, and actually goes to the meet with her. When GG sends out the mass text to everyone when they arrive, he was "closing the tab," making him one of the few people unaccounted for who could have sent it.
 * He was the only one who knew that Diana had stolen everyone's phones at the Jenny Packard show.
 * Remember back in Season 3? He said to Serena, "Threesomes, love triangles, affairs with married people... It seems as though everyone's problems are within my areas of expertise." Precisely. EVERYONE gossips to him. He learns everything firsthand.
 * The person who Nate is currently dating usually don't get hurt by Gossip Girl unless it's some form of infidelity, like Serena with Dan and Blair with Chuck. In A Thin Line, Nate sends out the tip that Chuck sent it because he wants to find someone who can confirm whether or not the story is true, something that he can only do by making it public. Needless to say, his plan works perfectly. Also, the most damaging blasts about Serena come out at the beginning of Season 4 (like the STD rumor), when Nate is the most angry at her.
 * He knew about Blair pretending to be a Columbia transfer and presumably told Chuck.
 * He was the only one not significantly hurt by the gossip bomb drops in the season 2 finale, even though GG's inbox had a lot more than Jenny's PG-13 photoshoot on him, like the fact that he had been squatting in his own house until his father turned himself in. Speaking of, isn't it odd that GG wouldn't have reported on that scandal a little more? The other upper east siders didn't hear a thing of it that wasn't directly from Nate.
 * In There Will Be Blood, Nate secretly knows that he and Jenny won't work out and, not wanting to hold her back from her dreams, sabotages their relationship by broadcasting the photo of them kissing for Dan to see. He later realizes his mistake and sends her a letter describing his true feelings.
 * In Last Tango, Then Paris, he already has the picture of Serena and Dan, which Jenny had sent in to GG, saved on his phone. Why would he have it if someone hadn't sent it to him personally? Remember, he had it saved before deciding to send it to Vanessa. Interesting, isn't it?
 * Also, he is the only one who never sent anything in to Gossip Girl (All The Pretty Sources). Considering the world he lives in where even Dan will send things in to GG, this is pretty odd. The reason: He doesn't need to send anything in as Nate Archibald, because he can edit gossipgirl.net at will. He was all set to release Gossip Girl's tips list because he knew he could just take over the Spectator eventually, giving him a gossip monopoly on Manhattan. It wouldn't hurt him at all. But why did he call Jonathon? Simple. Jonathon is his tech guy. That's why his phone was the one that beeped in The Goodbye Gossip Girl. Nate called him here to have the "tips" website set up to post to the Spectator.

I rest my case.

Dair has been the endgame all along
At least in the television show.

Over the five seasons, there has been a very slow, but very natural progression in Dan and Blair's relationship from one of mutual disgust to a legitimate friendship with romantic undertones and a possibility for more. Dan and Blair's personalities have always been more compatible with each other than anyone else and even when they "hated" each other, they both felt genuine guilt when one of them did something that ended up hurting the other. Serena, Chuck and all of their other love interests were just bumps in the road to a happy Waldorf/Humphrey ending, but the audience (unexpectedly) reacted very positive to the other non-Dair ships, so some Executive Meddling forced the writers to go more in that direction. The writers, though, still with their endgame in mind, slowly destroyed both of those relationships and any of the others either Dan or Blair have been in so that it's clear to anyone who objectively watches the show that Chuck and Blair and Dan and Serena are not meant for each other. The whole Blair/Louis relationship still has to play out, and if the pictures of Leighton Meester running around New York in a wedding dress, looking upset are any indication, I don't think that's going to end in a happy princess marriage. Plus, Penn and Leighton seem to think Dan and Blair belong togther.
 * Well, Penn does. Leighton can see the appeal of both Dair and Chair, according to her interviews. And Ed believes it should be Chair. (So, pretty much exactly like their characters' individual views...)

The reveal of Gossip Girl is a Red Herring.

 * The episode 100 reveal of  as Gossip Girl doesn't hold up against earlier episodes and characterization. Instead,   stepped in after Gossip Girl's absence after the car crash and is using the name for some nefarious purpose of her own.