Although this application does do a few extra things, such as nudging the overlay with the keyboard, customizing the overlay color and copying the dimensions to the clipboard, the same basic functionality is already built-in to the Mac OS.

When you use the keyboard shortcut Shift-Control-Command-4 from within any application, on any version of Mac OSX, the cursor will change into crosshairs with a live display of the x and y location of the cursor. Click and drag to create a selection area, and the x,y location is replaced with the width and height of the area in pixels.

Release the mouse, and the selection is copied to the clipboard. Use Shift-Command-4 if you want to save the selection image to disk.

I realise this app is only $5, but in my option I don't think there's $5 worth here.