I believe Civilizations 2 had been mentioned. If you want to have a go at that...Google "Freeciv" or "C-Evo". Both are fan-made knockoffs, and free to play.
Speaking of Slitherine Games, they also put out "World War II: Order of Battle". The boot camp campaign is free to download and play, but the rest of the games are DLCs you pay for, but its worth it. There's a Winter War campaign, as well as Japanese, German, Soviet and American.