Stephenie Meyer's site
Why It Rocks: All credit to Meyer for providing a regularly updated destination for fans that includes lots of insight into her work (she even published the abortive Midnight Sun chapters after the leak in a classy move) and more.
It's fairly basically designed, but that makes it easy to use (JK Rowling's for example, is flashier, but a little more awkward).
Twilight New Moon MySpace Profile
Why It Bites: Um, it's on MySpace? Okay, that's a cheap shot. Sometimes MySpace gets good trailers. But the pages are awkward and overloaded and the New Moon entry is no different.
Mostly, like most official presences, it's just exhortations to buy stuff. Consume! Obey!