I'm a huge fan of Zoomin, and have promoted it before on my site and I use it ALL THE TIME in Presentations.  Bryan Ressler sent me a link to his UltraMagnifier .

Some features:

• 1, 2, 4, 8, 16, or 32-times magnification
• Full multiple-monitor support
• Gridding of pixels (to facilitate counting pixels or checking alignment) in white, black, gray, or any user-defined color
• Update on a timer, on mouse movement, freeze, and timed-freeze modes
• Pixel-color display with choice of display formats (and color swatch)
• Save magnified image as BMP or PNG
• Copy magnified image to clipboard
• Printing with Print Preview
• Window features such as Stick to Screen Edges, Stay on Top

It's got everything that Zoomin has, and more!  Very nice.