Reticle
Reticle is an extension for Google Chrome that lets you drag and resize a ruler overlay to quickly measure the pixel dimensions of anything on a webpage.
Inspired by the excellent Mac app, Crosshairs, I created Reticle as a cross-platform solution for taking quick, precise measurements of anything on a webpage. As any frontend developer can attest to, this ability is extremely helpful for crafting pixel-perfect interfaces and page designs. My goals were to keep the extension minimalistic and lightweight (no JavaScript library dependencies), nonintrusive (requires no special browser permissions), and super quick and easy to use (lots of keyboard shortcuts).