Pixmicro is a digital microscope / webcam viewer with some extra features to make it useful together with a camera guided PCB drill jig described at http://kavionic.com/blog/Building_a_camera_guided_PCB_drill_jig.
The main features that makes it useful together with the jig is support for a movable cross-hair and a circle detection feature optimized for detecting holes etched in the copper of a PCB or drilled through it.
Whenever the cross-hair is placed over a hole in the PCB a circle is drawn where Pixmicro think the edge of the hole is. Pressing a "Calibrate" button will center the cross-hair in this hole. It also render an aiming guide to help you center the hole where the drill bit will enter after calibration.
A more detailed description as well as a windows installer can be found at http://kavionic.com/blog/Pixmicro.