I started playing Tribal Wars in 2008 and was hooked, now I just can't stop  playing 

Innogames
Tribal Wars started as a hobby among friends in 2003, Tribal Wars players numbered several thousand within a year, after 2 years a new world was started and the number of players had grown to 50 thousand, today there are 38 million players worldwide. Innogames was founded in 2007 and has 60 million players in its mmo games.

Why so popular
Tribal Wars is a text-based strategy game with very little graphic action involved, it is played in real time with players worldwide, there are chat forums and in-game mail, communication is a big part of the game.

The game is a browser based game, so, no software is needed, just a computer with a web browser.

Tribal Wars is free, you can purchase a premium account which has many additional features.

Starting with a single village which you build up and inhabit with various types of troops, you can conquer other villages and grow, the beginning of the game is not fast, your first village will take a couple of months to be fully operational, a world can last for years before there is a winning tribe.

Enhance
There are many web sites which supply scripts and other enhancements for TW players, improvements in TW (version 7+) reduces the need to use so many scripts. Tribal Wars Stats offers statistical info about worlds, tribes and players and offers a range of useful tools.