PixelZoomer by Matthias Schuetz

PixelZoomer is a Firefox extension for web developers which allows for zooming into websites. The core features are characterized by tools for pixel perfect measuring of distances and picking colors (HEX codes) with an eye dropper. The viewable area of a web page is being displayed in PixelZoomer through using a screenshot so that the view is based on real pixels.
The add-on ought to replace the step of using an image editing software when developing a web project. This provides the possibility of analysing sizes and distances directly in the browser. PixelZoomer uses a new CSS attribute to display the zoomed image. So the pixels are rendered crisp without using anti-aliasing.

Visit the PixelZoomer website for more details and a video demonstration.

Since version 2.0.0: PixelZoomer is now a WebExtension and also works in Firefox with Electrolysis (e10s).