VASSAL: cross-platform (at least Mac & Windows). VASL is a specialized version for ASL. Free.
Cyberboard: Windows only. Free. Popular.
Aide de Camp: Windows only. Pay only.
ZunTsu: Windows only. Free.
ACTS: Focuses on card-driven games like Hannibal: Rome vs. Carthage and We the People. Also supports Republic of Rome, which although it uses cards, isn't really in the same "family". Web based, so it seems to be completely platform independent.
By the way, there are also RPG equivalents. The main ones of which I'm aware are Obsidian Portal and RPoL.