Zoom it is a free software to zoom in any screen or annotate on screen. You can zoom in any corner of your screen with it. It shows you the detailed zoomed view of your selected screen. You can assign any hotkey to activate it. It also lets you to draw on zoomed window. Also have some other features.

The first time you run ZoomIt it presents a configuration dialog that describes ZoomIt’s behavior, let’s you specify alternate hotkeys for zooming and for entering drawing mode without zooming, and customize the drawing pen color and size. I use the draw-without-zoom option to annotate the screen at its native resolution, for example. ZoomIt also includes a break timer feature that remains active even when you tab away from the timer window and allows you to return to the timer window by clicking on the ZoomIt tray icon.