One of the best free tools available, not only it measures screen area in pixels but also can be used to do plenty of other useful things. Even better, Meazure is totally free!
• Measure screen area using a number of units including pixels, points, twips, centimeters, or user-defined.
• Capture arbitrary portions of the screen.
• Magnify the screen.
• Read the colors from screen, just like HTML color Picker.
• Display a grid overlay on the screen
• And many more.
Download Meazure from this link.
JR Screen Ruler
Another free utility for Windows that lets you measure screen area in various units including Pixels, Inches, Picas or Centimeters.
Download JR Screen ruler form this link.
Pixel Ruler v4.0 Is Very lightweight tool (605 Kb) for windows that measures screen area in pixels. Pixel Ruler is free to use up to 50 measurements and after that it prompts you to purchase the license. You can still use the free version of Pixel Ruler with no time limit and the license is not mandatory though.
Get Pixel Ruler v4.0 form this link.
Working in web browser and want to measure screen area in pixels of webpage, MeasureIt is a perfect solution for this situation. It comes as an add-on for Firefox, Safari as well as Google chrome. As the name suggests, it can only be used to measure the screen area and has no other feature.
Go to this page to get this add on for your browser.
Last but not least is the “Web Developer” tool kit. S the name suggests, it is basically a fully functional, free and pretty handy tool kit for web developers and it also includes pixel rulers as part of its features. It also comes as add-on for Firefox and Google chrome.
Go to this page to get Web Developer for your browser.