Lieutnant Skat (executable lskat) - from the German game Offiziersskat - is a card game for two players written by Martin Heni which is included in the KDE kdegames package. It's the card game Skat except that it's only for two players and somewhat simplified rules.
Lieutnant Skat allows you to play against the computer, a local adversary or a remote the adversary. Playing against a remote opponent requires that one of the players can open a (configurable) port for incoming connections. There is no central server or automated way to find random advesaries, so you have to IM or mail a friend and ask them to connect to you (or allow you to connect to them) to play. German opponents are a bad choice (or good, depending on how they see it) because Skat's a common card game there, so you basically don't stand a chance playing against them if you are relatively new to this game.