There's also Cyberboard, which is for email games only. Vassal can work both email and online.
http://cyberboard.brainiac.com/
An important caveat is that you need to own the games in question, the programs are only maps and counters and some charts. They are "dumb"-- they cannot interpret the rules, so you can move your 4-movement point unit 18 hexes if you like. If your opponent doesn't catch it, you can cheat real easy. OTOH, he can see your moves as they go, and call you on it.
I'm in a 3-year old game of Federation & Empire on CB, a 2-month game of GDW's Assault on Vassal, and about to start another Europa game on JET. I've played Flattop, GDW's Third World War, and some others on CB.