JRuler enables you to measure pixels on your screen. There's nothing to install; just unzip and launch this tiny program to see a resizable ruler you can use to measure anything on screen. You can adjust the size of the ruler itself, rotate it 90 degrees, and set it to display pixels, inches, picas, or centimeters.
Nattyware's Pixie is a colour picker that includes a mouse tracker. Run it, simply point to a colour and it will tell you the hex, RGB, HTML, CMYK and HSV values of that colour. You can then use these values to reproduce the selected colour in your favorite programs.