For Play by Post style games we have a unique web map that is specifically designed to run PBP games.
1) Pure web based, no installation required you just need a web browser.
2) creating, changing and viewing the map is really easy.
3) It's free.
4) It persistently hosted so doesn't require anyones computer to be on and can be accessed 24 hours a day.
PM Than (i.e. me) if you want me to set up a web map for your game and/or you want to use this forum to run your game.