SuperPower 2 is a newish "god-sim" game - I saw it in a shop for the first time yesterday and bought it. It allows you to take political/economic/military control of any country in the United Nations; to set your own gameplay objectives; to be as reasonable or unreasonable as you like (want to declare minority languages illegal? No problem. Want to change the political system? What sort would you like? Want to buy your struggling Third World country four squadrons of front-line fighters? ... well, you get the idea.). The writers have provided support (!!!! Can you imagine Microsoft doing this?!) for you to produce your own modifications - at the level of data, or at the level of a game. And yes, you can apparently play across the internet - though I have only just started exploring it.
Two small negatives. First - a 64MB graphics card (128MB recommended) just to have a spinning globe running whilst you play (most of the actual play relates to data not flashy graphics)? A bit gratuitous, I think. Second, the only start date (without modification) is currently 2001. Some others would be nice.
Otherwise - well, just remember you have other things you need to do as well.