IMO:
Harry Potter: Neutral Good. Likes things orderly, but not strict; willing to bend or even break the rules when necessary, but not without cause. Despises evil and all it stands for.
Ron Weasely: Starts out Chaotic Good like Fred and George, ends the series Neutral Good thanks to Hermione's influence.
Hermione Granger: Starts out Lawful Good, ends the series Neutral Good thanks to Harry and Ron's influences, plus first-hand experience that order isn't always better.
Lord Voldemort: Chaotic Evil and Vile. As mentioned elsewhere, everything he does and "believe in" is simply a front for his own desires. Chaotic Evil does not mean Chaotic Stupid (though he's got plenty of that, too). Promises are never made to be kept, lies slide from his mouth like a forked tongue from a snake, and even his least whim is usually indulged, save when he has more important matters to deal with (like his desire to kill the child in costume on Halloween... the only reason he did not do it is that it would have given him away to the Potters, and it was merely a whim!)
Malfoys: Lucius and Narcissa were Lawful Evil, with Neutral Evil leanings, while Draco was Neutral Evil. Note that while all Voldemort's followers were Evil, few were Vile. I'd say definitely Avery, Dolohov, Mulciber, Nott, and Rosier (the earliest Death Eaters), plus (probably) Fenrir, Bellatrix, Barty Crouch Jr., and the Carrows were Evil and Vile, while the other Death Eaters were merely Evil...
Severus Snape: I think in his childhood Snape was merely Neutral. Then, through influences in Slytherin and with bad experiences with James Potter and the Marauders he turned Neutral Evil. After the slaying of Lily Potter he saw the error of his ways, and was Neutral once more, with leanings toward Good. If he was pure Neutral he'd have never had a problem performing some minor Evil jobs, but in almost every case where we think he's done something Evil (like zapped George during the chase from the Dursely's), it was in fact a Good act, or a Good act gone wrong. If he'd been caught purposefully stopping another Death Eater, as he had done, he would have been killed out of hand. So I think by the end he was Neutral with Good tendencies.
Oh, and...
Dumbledore: Neutral Good with neutral tendencies. Bends rules happily, breaks rules when necessary, prefers order over chaos but values individuality over regimentation. Definitely good, as otherwise he could not have Fawkes, but is willing to recognize that at times less-than-perfectly-good actions must be taken.