Sid Meier's Civilization IV is the toy that allows the creation of fantastic mods.
Here you can find info about Rhye's mods for Civilization IV (including Warlords and Beyond the Sword) and download the new patches).
Rhye's and Fall of Civilization
Rhye's and Fall of Civilization (in short, "RFC") is the most popular mod made by Rhye. It was released in the Civilization IV expansion "Beyond the Sword", but it is compatible with all version of Civ4.