Marble has a simple plugin to measure distances. Extend it such that the edges (line segments) of the polygon it draws have three alternating colors, e.g. red, green, blue. Show the distance of each line segment above its center. Use a semi-transparent circle as a background for each vertice (node) to have it stand out against light backgrounds.
Patch at http://www.google-melange.com/gci/task/view/google/gci2012/7971212