OK...I was planning on the Akropovic but then I did some searching and i found this...Which I think looks/sounds Fuckign Sexy...(I have a thing for Loud)
http://motowerkz.com/Home.html
But it basically comes right off the Cat and pops out by the rear tire and eliminates the undertail completly...But it takes off like 15 pounds from the bike...And I like the look.