As a fan of online strategy games and have been playing many of them, there are some classic ones that are still strong among new games emerge in endlessly.  

1. Travian
Official Site: http://www.travian.com/

Travian was originally released in 2004 by a German team and quickly gained an international following.  Today, the game is available in over 40 languages and has over 250 servers worldwide servicing 5+ million players.  While the gameplay mechanics are similar to another German browser based strategy game, Tribal Wars, the two now offer very different gaming experiences.  The most recent version of Travian (version 3) was released in 2006 and offers several new features including heroes, higher building level caps, and the ability to annex oases territories which previously only served a visual purpose.

2. Tribal Wars
Official Site: http://www.tribalwars.net/

TribalWars is a browser based game and was released in 2003 as a text based game. The game is currently one of the most popular browser games in the world, ranking among the top ten in number of players. There are over 34 million registered users. Tribal Wars is exactly as the name suggests – an all out war between various ‘tribes’ and clans in a free to play web MMO. Every individual controls a village, and must struggle to expand it in order to survive. Set in the medieval ages, it is through the sword, shield and bow that each player will set out to conquer another – in real time.

3. Grepolis
Official Site: http://en.grepolis.com/

Grepolis is the third browser based MMO release by the German studio InnoGames. Like their flagship title, Tribal Wars, Grepolis is a strategy game where players start with a single city and build it into a sprawling empire. The setting this time is not medieval Europe but the cradle of Western civilization, ancient Greece.