Submitted by Tim Brown
There are also a couple of "what if" add on programs available - one of which titled " Doomsday" , which is basically an extension of the scenerio out to '63 and examines nuclear conflict. Germany nukes NY, that sort of thing. The other program is a alternate reality type scenerio in which for instance the US Civil War ended with both sides in a stalemate and WWII breaks out. Both are great additions and add playability.
ED -- The add-ons are HoI2: Doomsday (pictured above) and HoI2: Doomsday-Armageddon. As Tim says, HoI2:DD picks up where HoI2 leaves of, including an expanded tech tree as well as improvements to game play and system mechanics. Both have the same low system requirements as the original HoI2.