Utility to measure width/size of location on screen?

PicPick Tools - All-in-one Design Tools

PicPick is an all-in-one software for software developers, graphic designers and home user.

It has an intuitive interface and simple, elegant operation. After starting the software, you simply select one of the tool from the PicPick Tools trayicon menu when you want. It features a powerful capture tool, image editor, color picker, color palette, pixel ruler, protractor, crosshair, whiteboard and etc. You can use all these tools freely....

Not built for this purpose but quite a number of screen capture tools do that.

E.g. FastStone capture, MWSnap, !Quick Screen Capture etc etc