In the Harry Potter books, Severus Snape is one of the most enigmatic characters, and it is extremely difficult for readers to determine where his loyalties lie. A note of slimy evil seems to accompany him in many of the books, even when it is clearly demonstrated that he has saved Harry's life or has assisted other witches and wizards. It is hoped that his true nature will be revealed in the seventh and final book, Harry Potter and the Deathly Hallows.
According to some readers, Snape is simply evil, and has been working at Hogwarts for many years on an agenda of his own. When Voldemort finally returns, Snape turns at last, after being assured that his master is powerful enough to protect him. A great deal of evidence supports this claim, starting with the fact that he told Voldemort about a prophecy which led to the death of Harry's parents. Other evidence can be mustered as well: he's constantly mean to Harry, he doesn't appear to be honest about all of his activities and doings, and in Harry Potter and the Half-Blood Prince, he kills Professor Dumbledore.
However, the argument that he is purely evil is too simplistic. J.K. Rowling has shown immense depth and maturity in her books, and it seems unlikely that she would make such a crucial character clearly and simply wicked. He is an iconic character because of his difficult nature: unlike Voldemort, who is obviously evil, and Dumbledore, who is purely good, Snape is enigmatic and difficult, like many people in the real world.
Additionally, he does several things which are inconsistent with being evil. Despite the fact that he has taught Harry Potter for six years, he has not killed Harry. Indeed, a dark witch charges him with working for Dumbledore with this very argument, asking why he did not kill Harry when he had the chance, before he knew that Voldemort was back and wanted Harry for his own ends. He may be acting in a cruel way to Harry Potter to keep Harry away from him in order to better protect him. Alternatively, he may be struggling with the constant reminder of his disclosure to Voldemort which led to the murders of Lily and James Potter.
He also helps Harry out when he can, often in crafty ways so that his assistance will not be noticed. For example, in the first book, Professor Quirrel tries to unseat Harry from his broomstick during a Quidditch match. Snape casts a countercurse to keep Harry safe. In the fifth book, he refuses to provide Veritaserum to Professor Umbridge when she requests it to interrogate Harry.
It is also clear that he has been assisting the Order of the Phoenix, often in ways which put him at serious risk. He returned to Voldemort when he rose again: some readers argue that this is proof of his being evil and ready to serve Voldemort. Others point out that he was reluctant to return, but did so anyway out of loyalty to the Order, knowing that a spy on the inside would greatly benefit it.
Finally, Dumbledore has repeatedly said that he trusts Snape. Dumbledore is a trustworthy character, generally portrayed as being the most wise and informed of all the people that populate Harry's life. The reasons for Dumbledore's trust are never made clear, although they may be made apparent in the final book. Dumbledore's faith may be based on very good evidence that Harry cannot yet find out about, for his own safety.
The greatest act of evil that Snape commits is the murder of Dumbledore in the climactic scene at the top of the astronomy tower in Harry Potter and the Half Blood Prince. This act may not be as evil as it appears: he is, after all, bound by an Unbreakable Vow to help Draco Malfoy complete the task assigned to him by Voldemort. Dumbledore undoubtedly knows this, and probably pleads with him to carry out his duty, not to save Dumbledore's life, especially since he may already be dying. Dumbledore is acutely aware that if Snape will die if he does not kill him because that is the consequence of breaking an Unbreakable Vow. Dumbledore may also have felt that he was more useful alive and at least appearing to work for Voldemort. It is possible that he is actually still working for the Order, and that he will appear on the side of good in the seventh book after all.