game fanatic... uber-geekand is really properly thought out as well as being Open Source and free both as-in-speech and as-in-beer. Unlike a lot of OS by-geeks for-geeks software, it's incredibly easy to use. Setting up a networked game isn't at all a black art, and it handles a ladder tournament and all the systems to support the social side smoothly and comprehensively. And it has various options for custom rules and yay.
The only down side is that the interface is mildly awkward. You have to download a program, which is unfortunately Windows only. It's very Windows GUI in the way it's set up, which is fine; the activation barrier is very low if you're prepared to download and install it. But what's not so fine is that the board is 3-dimensional in a completely unnecessary way. I think the 3D part is partly because the Fugate is showing off his ability to handle 3D graphics, and partly to get round copyright issues. The fact is that the graphics don't work properly on my old computer (this is a known bug, apparently), so I have to rotate the board so that it appears horizontal (ie perpendicular to the plane of the screen!) in order to click on it. Which is a mild pain.
But I'm not in the least complaining, because yay networked Settlers! In the past I have also played Newman Settlers, which plays in Java embedded in the browser. That does have the advantage that it doesn't require a download, but it's basic rules only, and it's a pain to find opponents and it generally isn't nearly as complete as Sea3D.
I am in gamer heaven.