I've seen some really great ones. Tess Gerritsen's The Keepsake comes to mind. It was basically a student film if I remember correctly.
The trailer for Kate Griffin's A Madness of Angels was also a really good one as is the trailer for Quirk Books' mash-up, Sense and Sensibility and Sea Monsters.
There are a ton of other great ones out there, but I was especially pleased with the trailer for Sarah Langan's about-to-be-released title, Audrey's Door. It's truly creepy!
And imagine my surprise when I was alerted to the trailer thanks to a Facebook posting by Fearnet!! Yep, the horror network posted a link to a book trailer. That's big-time in my book!
Audrey's Door has been on my "To Buy" list since I first heard about the impending release, but now I want to run out and see if I can find it early (set for release 9/29 and I'm writing this on 9/24). There's an official Audrey's Door site and you can visit Sarah Langan's site for more info (and a special offer) as well.
And here's the fabulous trailer for your viewing pleasure: