For sometime now I have been using a handy tool from Mio Factory. It's a small non-intrusive set of high quality tools that resides at the top of your desktop and is always visible. Features include: Horizontal Ruler, Vertical Ruler, Color Picker, Screen Capture, and a Magnifier. All tools come in a set that is free to download and free to use forever.

Check out Mio Toolbox at http://www.mioplanet.com/products/miotoolbox/
You won't believe the stuff that they have on their website.