The requirements mention Windows or Mac but, although in freecol.org there are specific packages for those systems, there's also a "Platform independent installer" in Java bytecode, so I think (can't test outside Windows myself) that being in Java the game can be played in any system with Java Runtime Environment installed, including Linux:
[quote name='http://www.freecol.org/download.html']Users that know how to start a java program can download freecol-0.7.3-installer.jar. Start the installation program by running the command: "java -jar freecol-0.7.3-installer.jar". This installer is recommended for Linux users.[/quote]
[url="http://java.com/en/download/manual.jsp?locale=en&host=java.com:80"]http://java.com/en/download/manual.jsp?loc...ost=java.com:80[/url]
Also as we said in the Request topic, I'm pretty sure this could be played over LAN as well as Internet, but again I can't test myself.