A plate with evenly spaced holes at a given radius that allow you to rotate a chuck and then hold it precisely at a given number of intervals. This can then be used for gear cutting, machining facets at precise points or for drilling holes in a cylinder end cover for fixings.
This image shows the plate mounted to the back of a lather spindle and with a pointed indexing bar.
As you can see in the above image I use a permanent marker to highlight the holes that I need to select, this makes it much easier when you are working.
Alternatively you can mask off all of the holes bar those that you want to use, again helps a lot when you are in the workshop.