Travian is a free, browser-based Massively Multiplayer Online Game (MMOG), in which you are the ruler of a small tribal village (Roman, Gaul or Teuton). The first part of the game involves resource management and building construction — think the first couple of hours of Warcraft — aaanddd that is as far as I have progressed.
The world is persistent, and couple that with it being browser-based, means that you can start building something, go surf the web for a while, then log back in and see how your tribe is progressing. This is a good thing, because building construction generally takes in the neighborhood of 30 minutes. The more powerful/important the building or unit, the longer it takes to build, the more resources it uses, and the more prerequisites it requires.
Once you have a good base set up, you can begin trading with/raiding your neighboring villages, or building new villages. There appears to be a finite amount of land, which means you need to sign up NOW NOW NOW!!!
I have two user accounts (which is expressly against the rules) : “Levendis” and “Zartog”. You can only have one user account per email address, which can be a pain unless (like me) you have a domain name with a catch-all email address, and (like me) you might be inclined to do this sort of thing, and (unlike me) you have time to play this thing all day.
Many instructions and helpful tips can be found at the Travian Wiki.
Remember: It takes a viking to raze a village.