...for us presenters are ZoomIT from the Sys Internals Team at Microsoft.

http://technet.microsoft.com/en-us/sysinternals/bb897434.aspx

It's free, available for download and the little gadget every presenter need.

It has three features:

1. Gives you the possibility to zoom in on the screen to show important areas during demos.
2. Gives you the possibility to draw on the screen to mark out important areas during demos.
3. Gives you a break timer, helping the attendees to see when you start again.

When ZoomIt is running you just use the Ctrl-key plus the above number-key to activate chosen feature. In zoom-mode you can move and freeze the zoomed area with your mouse. In draw-mode you can draw free lines, blocks and arrows, with key-combos, on the screen.