I am currently involved in several wargames, all play-by-email. Steel Panthers has a very poor PBEM setup but is lots of fun.
Mike McMain runs a Ladder for several games, I am involved with the World at War games, Operation Crusader, Stalingrad, and America Invades. Check it out below.
SSi's Age of Rifles is coming. I'm sure that will be another good one for PBEM.