(because I'm the kind of person who not only cheated on every test to be placed in Slytherin, which I think qualifies me anyway, but actually cares enough to make this case)
Slytherin House gets a pretty bad rep. Personally, I blame He-Who-Must-Not-Be-Named for this. One bad apple spoiled the whole bunch for a little while, but I think we've got our own back. What does the Sorting Hat say about Slytherin? Does it say 'You must be this evil to ride'? No it does not. It says and I quote (because I can):
"Or perhaps in Slytherin
You'll make your real friends,
Those cunning folk use any means
To achieve their ends."
Cunning is the main word here. Not evil. Not meglomaniac. Not horrible hair or no nose. Cunning is the major qualification for Slytherin. And what's wrong with being cunning? This is like hating on Captain Kirk for changing the parameters of the Kobayashi Maru test (told you, serious geek here). In general, we may not be as wise as Ravenclaw, but we can find the back door to any solution. Sometimes even the side door.
I give you Draco Malfoy as an example. Now sure, you could love him just because he's really really really ridiculously good-looking (which, I learned at Dragon*Con, if you say to Tom Felton will earn you a laugh and humble 'I don't know about that'), but he's more than that. Take Half-Blood Prince. Draco's eluding Harry at every turn, always one step ahead. He's being cunning. While Harry's persistence in following Draco (don't get me started or we'll be here all day) is how he catches him because Draco's being (say it with me now) cunning. And no one can say Draco Malfoy is evil. Mean, yes. But actual evil? No way. He couldn't kill Dumbledore and let's not forget his clinging to Harry like a damsel in distress (seriously, don't get me started) in the final book because he's fighting against the Death Eaters. Oh, redemption.
Sure, some wackadoodle evil has come out of Slytherin, but this House has been around for hundreds of years and until Voldie, no one else was trying to take over the friggin' world. Hitler was a bastard, doesn't make all Germans before and since Nazis now does it?
So, I say, Slytherins let your green and silver flags fly! Smirk all you like, because while the Gryffindor's (which is hard to spell) are saving the day and the Ravenclaw's are making the grade and the Hufflepuff's are all singing "Kumbayah", we're the ones sneaking into the kitchen for extra pudding. And isn't that what it's all about?