Dimensionizer is a contextual menu plugin for the Finder that allows you to discover the dimensions of virtually any image in a single click.
Best of all, you don't even have to remember the dimensions to use them. Just click on the menu item representing an image, and Dimensionizer will copy the dimensions to the clipboard. Ready, set, go! It's that easy.
It is an indispensible tool for people who work with a lot of images, and especially for Web developers.