Travian

Travian was released way back in 2004 and has since expanded to include support for over 40 languages and is hosted on over 250 servers worldwide. With an estimated player count of over 5 million players, this free to play browser MMO RTS is easily one of the most popular around.

As with other RTS games, the main purpose of the game is to gather resources and materials so that you can expand your empire. Most activities within Travian cost resources, such as training units or constructing buildings. Different resources are harder to obtain than others, depending on where you start and your relations with other players, the resources you will need to gather are Lumber, Clay, Iron and Crops.
Players get the choice of playing 1 of 3 unique tribes. Romans, Gauls and Teutons. Although players are free to choose, each tribe does have an associated difficulty level. The Romans are the preferred choice for new players, they offer balanced defensive and offensive capabilities and can train some of the more powerful units in the game. The Gauls are for the average player, which a heavy focus on defense, and the last tribe Teutons, is the most difficult to play as they have very high defensive abilities but lack in certain areas of offense.  

The most appealing aspect to Travian is easily the Wonder system. Almost unheard of in browser games, Travian actually has an end-game objective. After 300 days an AI controlled army called the Natars attacks the game world in an attempt to wipe out players. In order to claim victory players must locate the blueprints for a Wonder construction and then attempt to build it while fending off other players and the Natars. At the end of the final war all player villages and resources are wiped, but they get to keep any gold that they've accumulated. This system is arguably one of the main reasons Travian has stayed so strong, even 8 years after release.