Terran Alliance: Modern North America. The Terran Hegemony is culturally similar but had a feudal government.
House Kurita: Feudal Japan.
House Davion: Medieval Britain and France.
House Liao: Imperial China with some shades of Communist China.
House Marik: The Holy Roman Empire during the Renaissance.
House Steiner: The German Empire prior to World War I.
Free Rasalhague Republic: Renaissance Sweden.
ComStar: The Catholic Church, at least initially. I'm not sure exactly what ComStar and Word of Blake would be equivilant to after the schism, though.
The Clans: 13th-century Mongolia.
Marian Hegemony: Roman Empire.
Republic of the Sphere: Roman Republic.
Anything I missed?
These are just the styles of government, there are obviously many different cultures within each state. Also, planetary and regional government often diverge wildly from the state they belong to.
word of blake is clearly modeled after extremist muslims, hence the whole "jihad". nothing against muslims, but you guys extreme fanatics are a little too much. at least the catholics just molest alter boys instead of blowing stuff up.