Cybernations is a fantastic browser game based around nation-building. You play as the ruler of a nation and you purchase infrastructure, land, technology, and improvements to grow your nation. The game itself is fairly simple for a browser game, though not as reductionist and boring as some others (see: Galava).
Cybernations is most interesting because of its political system. Of all the MMOs I've heard of, Cybernations easily has the most developed and interesting political interactions (with the possible exception of Eve: Online). The game has a violent, tumultuous, and fascinating history. Individual nations mirror the citizens of a nation, and alliances (groups of nations) mirror nations. Each alliance has its own distinct culture, political philosophy, treaties, etc.
A large part of the game is played out on the game's forum and other off-site forums, along with IRC. This is where a good deal of the politics takes place. A good deal of people take the game pretty seriously, so don't be surprised to see yelling, threats, and general silliness. It's very difficult to describe all of the complex machinations here, so I'll just let you guys figure that out as you get into things.
Advice: If you're new to the game, you need to know a few things. First, you're going to receive a ton of recruitment messages right off the bat. You want to join an alliance because there are a lot of mean people out there who will beat you up if you don't, and also, the communities are really what makes the game great. Do not choose your alliance based on how much money they promise to give you. Choose an established alliance that meshes well with your ideas of government. You can use the wiki (
http://cybernations.wikia.com/) to look up an alliance's government and history.
Infrastructure gives you population, and is the main thing you'll buy. Technology helps with your income and military, but you shouldn't focus on it until much later. Land is also secondary. Only purchase land when your population density is above 100 citizens/square mile. You want to have 30% of your civilian population as soldiers during peace time to act as a police force.
Basically, it's a lot of fun, and if you enjoy politics, backroom dealing, war, shouting matches, or just hanging out, it's your kind of game.
http://www.cybernations.net/default.asp?Referrer=VilienUse that link if you'd like to give me a little boost in-game, but it really doesn't matter anyhow.
If you have any questions, comments, etc., I'm here.