Eye Dropper - A simple color sampler tool
Image Properties Context Menu - You can view image properties with the right mouse button
PerfectPixel by WellDoneCode - You can add a png overlay of the site you are building for reference to achieve pixel perfection (or as close to it as you can get)
ShowIp - Simply shows the website's IP address
View Background Image - This one shows the background image of some elements. Unfortunately still doesn't work as well as in Firefox
View Image - View the image in a new tab.
Web Developer - Useful for a ton of reasons :-)
WhatFont - This one tells you exactly what font is used under the mouse pointer. Also tells the size and color...