eRepublik

eRepublik is a massively multiplayer online game, social networking and strategy web browser game developed by eRepublik Labs. The game is set in a mirror world (called the New World) where players, referred to as citizens, join in local and national politics, set economic policy, start businesses and wage wars with other countries.
This is one of the best web-based MMO I ever played, I really suggest you to play it.
http://erepublik.com