(Another under the banner of "Whose Side Is Snape On?")
The number one reason I really think Snape is good? It just makes a better story. I mean, what's fun about a guy who is mean, looks evil, and, in fact, really is evil? A misunderstood and maligned guy, that's really good at heart--so much more interesting.
But, aside from that, I discovered what I consider to be the first piece of really, good, solid, canon evidence in this wonderful essay by Edmund Kern, entitled "Snape's Eyes". Are you ready?
As you all should know, at the end of Goblet of Fire, Barty Crouch Junior acting as Alastor Moody takes Harry back to his office. Just as Barty-as-Moody is about to AK Harry, Harry sees three faces appear in Moody's foe-glass.
"Harry . . . saw Albus Dumbledore, Professor Snape, and Professor McGonagall looking back at him out of the Foe-Glass. . . . [Dumbledore] stepped into the office . . . . Snape followed him, looking into the Foe-Glass where his own face was still visible, glaring into the room." (GOF, 678-79)
For Snape to be a foe of Barty-as-Moody, he would also have to be a foe to Voldemort. Via Snape's dark mark he would have certainly known that Voldemort was back by that time (he would have felt the summons, same as the other Death Eaters), and thus would not have been Moody's enemy if he was on Voldey's side.
Snape is good, Dumbledore is infallible, and all is right with the world.