GMScreen's primary goal is to allow a game master to display a 'to scale' map to the players using a video projector to project the map onto the gaming table. Although player icons could be easily added (and you are free to do so), the program was created to work in conjunction with physical miniatures that the players can move just as on a printed battle mat on a hex/square grid. Once configured, the program will properly scale maps to have, for instance, one physical square inch representing each grid square.
GMScreen is cross-platform with scripts included to run from both Linux and Windows. If anyone would like to contribute a script to run it from a Mac, it would be a great addition and could be included in the next release.
Free maps for use in your D20 or other campaign can be found here.