This plugin will take a set of points (specified with the Path tool) from which it will generate a Voronoi diagram in the form of a new path. There is the option to preserve the original points as a separate path, and a hack is used to ensure these original points can be stroked (assuming a round or square cap style). The algorithm used is described here:
This plugin creates a new entry in the Paths dialog right-click menu.
Be sure to read the readme.txt file for installation and user help.