Borders
This is for doing some Islamic tiling inspired math art. They frequently use the technique of outlining stars or polygons to get an overlapping or weaving approach. This sketch has my attempt at making a tool to do that.
The 1st tool is Outlines, and will draw two lines parallel to a given segment (the endpoints, actually) at a given distance.
The 2nd tool is BorderSegments, which draws segments on those lines at just the right length to dovetail with the segments that come before and after. (That's why you have to select 4 points, and it draws the segments bordering the middle two.)
See also: http://www.geogebratube.org/material/show/id/51256 which has a star making tool.