On the eve of the release of 7th and final Harry Potter book I wanted to go on record stating with some certainty that Snape is a Death Eater.
My theory is based on the way he treats everyone around him-- with loathing. Snape loves the Dark Arts, and talks about Voldemort with something near reverence. When Harry calls him by his name, Snape tells him to shut up. Snape himself refers to him as "The Dark Lord".
As further proof of Snapes allegiance being with Voldemort, I cite the following:
During his meetings with Harry in which he attempts to teach Harry Legilimancy. Harry asks if Voldemort can read their minds at that moment. Snape tells Harry that he cannot because... "The Dark Lord is at a considerable distance..."
How did Snape know this? Unless he knew Voldemort’s location?
Later, he comments, "You seem to have visited the snakes mind because that is where the Dark Lord was at that particular moment. He was possessing the snake at the time, and so you dreamed you were inside it too."
"How do you know?" said Harry urgently. "Is this just Professor Dumbledore guessing, or...?"
Snape responds, "It is enough that we know."
How did Snape know that the Voldemort was possessing the snakes mind at that moment unless he got this information first hand from Voldemort himself?
Later, Harry, reflecting on his dream about the snake attacking Mr. Weasley, asks what’s in the department of Mysteries?
“What did you say?” Snape asked quietly, and Harry saw with deep satisfaction that Snape was unnerved. “And why would you ask such a thing?”
Why would Snape be ‘unnerved’ that Harry would ask about the Dept. of Mysteries, and dodge the question unless he knew that Voldemort was searching for something in it? And didn’t want Harry to stumble upon the truth?
I believe that this conversation between Harry and Snape is very revealing about Snape's true character, and where his allegiance lay.
In roughly 24 hours we'll know the true story of the fictional character. It should be interesting.