Laser Squad Nemesis
Apparently, this game has been getting very good reviews. It is a two player, play-by-email game that is currently only available for Windows. Its creators are also the designers of the X-COM series.
You can download the game and play 3 games for free, as well as play “hotseat” games where you switch seats with a 2nd player. Subscriptions are $25 for 6 months.
Anyone know of other play-by-email games?