I think several games like that exist, of varying complexity. Sorry I don't have names ready :-(
I know WeeWar has an API, but that is probably not the kind of game you have in mind. I think you mean long running simulations with agents trying to survive.
There might even be one by Microsoft, not sure if they are language agnostic, though.
I think there is still room for interesting worlds of that kind.
The Microsoft one you are thinking of is called Terrarium. It was built to help people learn .NET 1.0 but I don't think it's been updated for newer versions of .NET (currently 4.0). It was .NET specific, meaning you could write your dll in any .NET language but non .NET languages would not be compatible.
I've seen WeeWar before but didn't know they had an API. Looks interesting.
I also wondered if Microsoft had anything in this realm. The thought of company sponsoring the server would be interesting as well. We might be able to get their attention if the central server was written C# and deployed Azure. I have experience with both technologies :)
I know WeeWar has an API, but that is probably not the kind of game you have in mind. I think you mean long running simulations with agents trying to survive.
There might even be one by Microsoft, not sure if they are language agnostic, though.
I think there is still room for interesting worlds of that kind.