Mac App - Pixel Measurement Tool

I would like an web application that you could download on the mac app store or websites. The app will be able to let you draw a straight line with your cursor on a frozen webpage and tell you exactly how many pixels/cm long the line is. (maybe a later software update could feature the user being able to draw a box and know how many pixels are along either side). The app could feature a button which would completely freeze your webpage so that you can't click on anything (other than the app icon to unfreeze) - this helps you draw the line without clicking on any links.

The software can be used in graphic design. If you want to make a banner for a webpage and you don't know the dimensions for the ideal banner, you could use the software to help you design a perfectly sized image.

The app could be downloaded through websites or put on the mac app store.

Anyone that thinks that they could design a software capable of this by all means get in touch.

Any questions please feel free to leave a comment.