Read on, if you dare:
Here's where I dork out on HP and my McGonagall theory. Throughout the books, Dumbledore says that he trust Snape. But Minerva slides under our radar: She's just present enough to be likeable (a crotchety old maid, so to speak) and there are things that could be clues scattered throughout the series. I'll start at the beginning. Page cites are to softcover U.S. for Books 1-3, hardcover U.S. for books 4-6.
* In Book 1, before we even meet Harry for the first time, she is waiting--as a cat--at the Dursleys. She is surprised, ruffled, to see Dumbledore at the Dursleys, and did not know he'd recognize her as a cat. So, why was she there? How did she, of all people, know that Harry was going to his uncle's, and she questions Dumbledore about whether Voldemort is really gone. P. 9-11.
Remember also her reaction to the celebrations of Voldemort's death: she's cold, sharp, and angry. These are odd reactions, no? And she finds it "astounding" that Voldemort couldn't kill a little boy and wants to know how. P. 12.
Was she there to intercept Hagrid? Did she know about--and seek to stop--the protection of Harry's mother's family? Another thought: How old is McGonagall? We know she's quite old--older than Sirius/Snape/the Potters/Lupin. In a 2000 interview, J.K. Rowling says that McGonagall "is a sprightly 70" and that Wizards have a much longer life expectancy than Muggles. (2000 scholastic chat). Doesn't that put her at school about the same time as Riddle? (Wikipedia says she was born in 1925). And couldn't we see her as the Gryffindor Prefect at the same time he was the Slytherin Prefect?
Also, recall that she is the one who makes Harry a Seeker--and directly puts him into the line of danger when Quirrel tries to jinx his broom. Isn't it interesting that one of the most rigid and rule-following characters in the book (she won't let him go to Hogwarts w/o a permission slip in Book 5, for example), is so quick to break the rules to set up Harry in what could be seen as an "accident"? Recall her reaction when Harry and Ron tell her about the Philosopher's Stone? She reacts with "suspicion and shock". Also, why would she be the one to send him the broomstick? Isn't that kind of weird--she would have known that he had money from his parents, no?
Also, also, recall that the chess game--McGonagall's protection for the stone--was intact when they got to it--and they left it shambles. How did Quirrell get past it? Finally, I'm fascinated by Quirrell's reaction when Harry confronts him. He says "Yes, Severus does seem the type, doesn't he? So useful to have him swooping around like an overgrown bat." Is that JRK's way of telling us that Snape is a red herring?
In Book 2, she's the one who finds the final message that says Ginny was taken into the Chamber, and tries to close the school. P. 293. Then, she says "weakly" that they'd all like to know how Harry and Ginny get out of the chamber and probes repeatedly for the answer. P. 327-328.
* In Book 3, McGonagall is mysteriously absent from the scene after Peter Pettigrew escapes. And she "goes white" when she says she "never speaks ill of her colleagues"--is she talking just about Trelawney there? (or even if she is, does she recall the prophecy)? P. 109. Moreover, doesn't she call Voldemort "the Dark Lord" when she takes Harry's new broom (a gift from Sirius) away from him? Harry later notes that only the Death Eaters call him that.
In Book 4, who is with Barty Crouch Jr. when the Dementor administers the kiss? McGonagall (and Cornelius Fudge). We know Dumbledore didn't want this to happen - why wouldn't his second in command, McGonagall, stop it? Also, recall that Voldemort says his "most faithful servant" was at Hogwart's. Is that Crouch? Too easy, I think. Sure, he's important in Book 4, but the story arc for him is short. Perhaps I'm too quick to dismiss the obvious, but I don't think it's him. Snape? I don't think so, because there are references in Book 6 to him having to go back to Voldemort after the hubbub dies down to sort out his role--he tells Narcissa and Bellatrix about this at Spinner's End. So at the point Voldemort says this, he didn't know Snape was coming back to him. So there must be another spy at Hogwarts. (At that point, Wormtail has already returned.) Note also that when Harry returns from the graveyard, McGonagall is one of the people trying to yank Harry away to the hospital wing. We know that it turns out badly when Crouch/Moody takes him away. Is it important that McGonagall has tried to do the same thing?
*In Book 5, McGonagall tells Harry about the importance of "keeping your head down and your temper under control". P. 249, 319.
When she's stunned by the ministry, which side is she on? P. 723-24. Note, however, that this is a hole in the McGonagall-as-spy theory: getting yourself stunned is not likely to be a stellar moment for a spy. And she's nowhere near the Ministry during the fight. Note also her response to Umbridge on how long she's been teaching at Howarts--doesn't that track fairly closely with the number of years since Riddle was there? I think that might (though I've not run the numbers exactly) help confirm she was at Hogwarts during the time Riddle was a student. Also, I feel like I remember that Madam Pomfrey somewhere in here comments on McGonagall's dueling prowess back during the day, but can't find the cite to it.
*In Book 6, Mcgonagall also digs for info when Katie Bell is cursed, and immediately dismisses Harry's Malfoy theory. P. 253-55.
As we know from later on, Malfoy is the one who started that curse--using the necklace from B&B. So McGonagall has--whether deliberately or unintentionally--thrown them off the track. Unless, of course, someone else has helped Malfoy. . . We know that Malfoy tells Snape the night of Slug's Xmas party that he has "other people" "better" than Goyle and Crabbe on his side. Next, after Dumbledore's death, McGonagall comes into the ward where Bill is. Harry tells her "Snape killed Dumbledore. " She says "Snape. We all wondered . . . But he trusted . . . always . . . Snape . . ." P. 615
Keep reading that passage and the next--it makes sense both with Dumbledore as "he" and Voldemort as "he." And, though McGonagall confesses she sent Flitwick to get Snape, she rather bizarrely can't recall the details of how he joined in the fight, and she recounts her tale "distractedly" P. 117. Isn't it an odd time not to remember what happened?
She is "curt" when saying Dumbledore would have been happier to think there is more love in the world. P. 624.
And then she tries to pry the info out of Harry about what happened, but he won't tell him anything in return. And she immediately suggests closing the school. P. 626-627.
Given the story arc, I think it's clear there has to have been a spy at Hogwarts and/or in the OoP. Snape's the easy answer: hate the guy who is mean to Harry. But isn't that too simple? McGonagall is the secret: we like her just enough throughout the books. However, J.K. Rowling herself says that McGonagall is an "old softy" and a "very worthy second in command"--but not Dumbledore's equal. (JKR's interview with scholastic in 2005, and 2000). Could be read both ways, no? Note also the sequence of cats Mrs. Figg has cats, Crookshanks, and McGonagall's an animagus. Moreover, in the movies--which we believe JKR has insight into, McGonagall is wearing green, as she does in the formal parties (whenever she's not wearing tartan).
Finally--one last thing: Wikipedia says the Queen of Spades depicts Minerva, AND spades in Tarot indicate obstacles, trouble, death.
Did I just blow your mind? Because that just happened. Fine work, K.S.