Online strategy game eRepublik is used by over 200,000 active players internationally. The site calls itself the "first massive online social strategy game" and invites you to "change the world and rewrite history. Over 1 million people have already done that, according to eRepublik. The eRepublik API--or eAPI, as it's called--can be used to retrieve game data in a developer-friendly format (supports both XML and JSON). The eAPI also offers functions to interact with the game's ecosystem.
Your application can access detailed information within the eRepublik game regarding regions, countries, wars, companies and citizens. You can also learn about markets, industries and battles.
The eAPI documentation covers a 17 page PDF.