The Maki App is designed to allow you to take your Photoshop design of a site layout and overlay it on to your actual web build so that you can match the two with pixel perfect precision. I've not tried it out myself but can attest to the fact that I've been asked to nudge items in development a pixel or two in one direction and it seems like Maki could help prevent that. It might be overkill but it's at least worth taking a look at.

Via Renaud.