Thumbs Up: Pixel Perfect Firefox Extension
Take a look at the Firefox extension Pixel Perfect if you implement webpages based off Photoshop mockups. It overlays a transparent image of your choice over the webpage so that out of place elements are easy to spot.
It's currently at version 0.6.1 but it has plenty of potential. I could see how it could be integral to the workflow but at the moment it's great for quick checks.