Sid Meier's Civilization V: Brave New World is now available for download through Steam (in the US - it'll be available internationally on Friday). It's downloadable content (DLC) that works with both the Mac and Windows versions of Civilization V, also available for download through Steam.
Here's what the developer has to say about the expansion:
Sid Meier's Civilization® V: Brave New World is the second expansion pack for Civilization V - the critically acclaimed 2010 PC Game of the Year. This new expansion provides enhanced depth and replayability through the introduction of international trade and a focus on culture and diplomacy. Your influence around the world will be impacted by creating Great Works, choosing an ideology for your people and proposing global resolutions in the World Congress. As you move through the ages of history you will make critical decisions that will impact your relationship with other civilizations.
Along with that, Brave New World adds nine new civilizations, eight new wonders, two new scenarios, four new gameplay systems and dozens of new units, buildings and other enhancements.
Civilization V is the most recent edition of Sid Meier's long-running strategy game series, in which you must uplift a human civilization from its roots as a stone-age hunter-gatherer society all the way to the modern era and beyond.