TripleA or Abattlemap are the best two options, and both are free, and both have many different game versions to choose from. TripleA is a full blown java application with the rules hardcoded and is typically played real time, although it has a save feature and a play by email option. Abattlemap is simply a visual representation of the map/pieces and can be played by forum, but has none of the rules hard coded into it and is the closest to playing the real board game.
http://triplea.sourceforge.net/mywiki for tripleA information. You must download the maps separately though the program, it is very easy.
http://www.flames-of-europe.de/ is where you can get Abattlemap, you must register to get it.
GTO is an evil, vile company that made a crappy product that they charge for, and then tried to get TripleA shut down. Yes, wotc only went after tripleA because GTO told them to do it, at least in my opinion. Otherwise they would have went after Abattlemap as well, and shut them both down with legal action instead of a simple C&D letter to honor there licenses agreement with GTO.