Memoir '44 is by far one the best and most simple tactical World War II board games.  It is now available to play online and is available as a free download either through there website or even Steam.  If you are a Steam user, I recommend getting the game through Steam as you can use your Steam account for there login system. 

You can play solo against the AI or against other players online.  The AI is surprisingly good.  The interface for the game is fantastic.  If you have ever played the Settler's of Catan game online, you know how bad interfaces can ruin a game.

The game is not 100% free.  They start you out 50 coins or "Gold Ingots" for free.  It costs 3 coins to play a single game of Memoir Online, so you can sufficiently try out the game without paying for at least 16 games.  Also it's only the person that starts the game that loses the coins.  If you pay for the higher level membership you can also create your own scenarios!  This makes the game a lot more interesting as other players are playing there custom scenarios online as well.

Every expansion is also available to play online.  Weather you are a seasoned Memoir '44 fan, want to learn how to play the game or even want to try out the expansions before buying them, Memoir '44 Online is a fantastic experience.  It is by the best implementation of an board game online I have seen yet.

Memoir '44 Online
Memoir '44 Online on Steam