Severus Snape

^ what I was thinking, exactly

Uh. Yeah.
Okay. Slughorn and Snape are "good" (though Snape is still classified as a giant jerkwad in my book). And Regulus turned out to be a pretty decent bloke too. And Draco never really had evil intentions. But 99% of em ended up went bad.
My point: even though Slytherin turned out a few not-as-bad-as-they-could-have-been blokes, most of em went really bad. A reputation is "a GENERAL ESTIMATION in which a person/thing is held by the public", meaning a reputation by no means represents the real thing. I think if you estimated, the majority of the wizards/witches in Slytherin practiced dark magic.
Therefore, the reputation upholds.

I think the biggest reason they have that reputation is because most Slytherins tend to be a bit selfish and snobbish. They are, first and foremost, worried about themselves and what will benefit them the most. But that doesn't mean they aren't capable of love or that they can't grow and learn to put others first. Snape, Regulus, Slughorn, and Narcissa are just a few of the Slytherins that helped hugely in bringing about the downfall of Voldemort. And of course there was poor little Draco, who really didn't stand a chance. He wasn't evil and he wasn't necessarily on the good side. He only wanted peace and to be left alone. I think that's how the majority of Slytherins feel. They're not all power-hungry jerks who live to make others suffer, they mostly just want to stay out of the war and be left alone.

look at Sev

Severus Snape. Narcissa Malfoy. Regulus Black.

Ok, while I do love Regulus and Narcissa, I think they're getting too much credit. They did not act selflessly, they did not do what they did for the Greater Good; Regulus died still believing that Voldemort was right in principle, but wrong in the execution of the principle. Narcissa did not give a damn about the Greater Good. The only reason she did what she did is because her true loyalty was to her son. The fact that the Malfoys abandoned the Dark Lord does not make them Good, it just means they're typical Slytherins: you put yourself first. She grabbed her son and wisely left, and her broken, useless, empty shell of a husband ran after them.

Yes, Regulus and Narcissa were very brave and acted courageously, and their actions contributed to Harry's victory, in the end, but I don't believe it makes them "Good" in any sense of the concept.

I think two of the best examples of Good Slytherins are Andromeda Tonks and Horace Slughorn.

Severus Snape, however...now, he's a different story. Dumbledore's own statement, made more than once throughout the epic story, proves this out. The essence of that statement being, of course, "Only a worthy Gryffindor can take the Sword." ;)